Usenet is the primordial soup which all modern internet discussion forums evolved from. Back in the dark ages it was used for actual discussion, and in some small secluded corners you may actually still find it being used that way, but that isn't why you're here.
Today, Usenet is primarily used for the distribution of 'binaries', which can be everything from real honest-to-goodness Linux ISOs to every kind of illicit digital commodity under the sun. Furthermore, Usenet tends to be one of the first places said goods arrive, shortly after topsites and IRC, and generally before BitTorrent. Usenet is popular in this respect because it was the first service to provide such a wide-scale distribution channel, and it has had staying power due to being decentralized and thus quite hard to shut down.
Boring History Lesson
Usenet is made up of a hierarchical system of newsgroups, sorted roughly by what goes on in said newsgroup. After The Great Renaming, we ended up with the broad top-level categories like comp.*, sci.* and rec.*; and then you had alt.*, the catchall for Everything Else. Binaries, so-called because they're not text, generally live somewhere within the great tangled mass that is the alt.binaries.* hierarchy. To start cruising through Usenet, there's a few things you'll need:
Newsreaders
To get files out of Usenet, you first need a program called a Newsreader. There are three kinds of newsreaders: Text-Oriented, Binary-Oriented and Pure NZB Processors. We'll be covering the latter two.
Binary-Oriented Newsreaders
These programs are designed for the old way of downloading binaries from Usenet. They generally require you to download large quantities of headers, read through all the headers, and manually pick and choose what stuff you want to download. All of these newsreaders have gotten with the times and can process NZB's, but they're still geared towards the Old Way.
NZB is an XML format developed by Newzbin to side-step the process of downloading headers. Instead of subscribing groups, downloading and then combing through hundreds of thousands of headers, you search Newzbin and (assuming you have premium credit) generate an .nzb. Simply hand that .nzb off to one of the following programs and you'll skip downloading headers for hours, and directly start leeching just what you want.
SABnzbd+ - (Python, freeware) SABnzbd is a Web-based self-contained NZB processor written in Python. It will effectively do everything for you. Given a Newzbin post id, it will download the .nzb, parse it, download it, verify it (and repair with par2 as necessary), extract it, and delete the rar set. Combine SABnzbd+ with Newzbin V3, Firefox, and the nzbdStatus extension, and the entire leeching process becomes point & click. Add in an RSS feed and you won't have to do anything, files will just automagically appear.
Ninan - (Java, freeware) Ninan is like SABnzbd but written in Java. Unfortunately it just isn't very good, and Java is likely to blame.
hellanzb - (Python, freeware) hellanzb is - again - like SABnzbd or Ninan, but whereas both of those were designed from the ground-up to have web interfaces, hella was designed to be a heavily extensible CLI program. Some extensions include Growl and Growl-type notifications, a variety of web interfaces, etc. There's also an unofficial Windows Installer, in case you don't feel like figuring out how to get all the Python dependencies up and running on Windows.
nzbget / nzbperl (Linux, freeware) two little CLI programs designed to just sit there and grab binaries described by .nzb files.
Other Software
If you do things manually, you're going to end up with a ton of rars and pars. So you'll need a few more bits of software. It also never hurts to have this stuff installed.
WinRar (Windows, shareware) - Nearly everything posted to Usenet is rarred. Not for compression reasons, but for compartmentalization. Due to the nature of Usenet, most files need to be broken up into multiple parts of so many lines to make sure news servers accept them. Since files have gotten large, it makes more sense to post a set of 10 rars broken into 50 parts a piece than one huge solid 500-part post. Back in the dark ages before Pars (described below), this also made it easier to request reposts, because then you could request rar #7 rather than the auto-generated parts 37-86. Nowadays, rars are mostly used out of tradition, since Par2 works just as well on solid files as it does sets.
QuickPar (Windows, freeware) - Pars are files which - using crazy voodoo magic - can reconstruct missing parts of an archive set. So if some section of the file gets lost on it's journey across the internets, with ample parity data you should be able to recover the set and go about your business. Par is the original format, and requires you to have "n+1" pars to recover "n" missing files in an archive set. Par2 is the newer format which has more or less completely superceded Par. It works off of "blocks" of arbitrary size rather than entire files, so even incomplete rars count towards "completed" data. Basically, if the poster used Par2, download everything available to you, even if it's incomplete. If the poster used normal Pars, skip any files that are incomplete and hope for the best.
Indexing Services
Usenet is a Big Place - like, terabytes-posted-per-day big - and sometimes people don't post things where you expect. These services let you skip all that.
Newzbin - Newzbin is the original searchable Usenet binary indexing service. For a 50c/week subscription you get the ability to generate all the nzbs you want, which are tiny batch files that can be used in most newsreaders to let you skip the time-consuming process of downloading headers, finding the files you want, selecting the files, etc etc. It cuts the process down to: get nzb, feed nzb to newsreader, wait for files to come. NewzBin is also human-moderated, so if you go to download something you'll know that what it says is what you get. It also has comments, so if a post is broken / corrupt / incomplete / fake / password protected or sucks for any reason whatsoever, you can expect that someone will have bitched about it in the comments. If you use Newzbin, SABnzbd+ and Firefox, I strongly suggest getting the nzbdStatus extension?, which allows you to easily and automatically send nzb's straight to SABnzbd+. Sadly, Newzbin is invite-only right now, if you hit up their irc server, you should be able to find someone who can help you out.
binsearch.info - BinSearch is a Usenet indexing site like NewzBin, but it's free and automated. This is a good and bad thing. Good because it's free, and we like free. Bad because it isn't human-edited. Usually the .nzbs you can create from binsearch will be incomplete to some extent, because it just isn't perfect at picking out what's part of a "set", but generally with pars you should be alright. If not, it's at least a good start.
alt.binaries.nl - Another free Usenet indexing site, more like Binsearch.info than Newzbin, due to lack of human moderation and not being able to pick out what constitutes a "set".
Other Options - It seems everyone and their dog wants to provide you with indexing these days for various prices. Some roll it in to their hosting services for free, like the searching systems provided by EasyNews and UNS. Others have it as added features for their software, like Newsleecher (Their "Super Search" package is $29.99/year, as opposed to just the program, which is $19.99/year) and GrabIt (GrabIt itself is free, but the integrated search feature runs you anywhere from $1.99/mo to $20/year). If your Usenet Provider supplies it for free, super, try it out. But if they don't, I still strongly suggest Newzbin over Newsleecher Super Search or Grabit Search. Neither of those services have editors to my knowledge, and without editors it's a pain in the ass to find some stuff on Usenet. Plus, you end up locked in to whatever client you went with. If I use Newzbin, I can feed it to SABnzbd on my PC, nzbperl on my Linux box, or give the resulting nzb to a friend for them to use on whatever crazy newsreader they want. If I used Newsleecher Super Search or Grabit Search, I'd be limited.
Usenet Servers
Many ISPs provide you with free Usenet access, either through their own server or through a third party. Determine this FIRST so you don't shell out for a service you don't need. As of late, more ISPs have gone to outsourcing their news servers. . . but this is a good thing, because they've been outsourced to companies that know how to run a news server. Comcast outsources their stuff to Giganews, and gives you a 2gb/mo account there, which may translate to a discount on a full Giganews account. Time Warner has recently switched their news service to an unlimited Newshosting account. Your ISP's Usenet access might not suck, so do some research regarding what they offer before shelling out for a dedicated Usenet provider. Newzbin provides a rather extensive list of what assorted ISPs worldwide offer with regard to Usenet.
If your ISP doesn't offer Usenet access, or if their access sucks, you'll want a dedicated Usenet Provider. When it comes to determining what makes a news server suck, there are four things to be concerned with:
Completion - What percentage of posts actually make it to the server? Because Usenet is decentralized, you can only download what files have shown up on your server. Sometimes this just causes lag, where it can take a few hours for a post to actually get there. Most times it results in the file just not showing up at all, which sucks because then you've got an incomplete binary and will have to use Pars.
Retention - How long do posts actually stay on the server? Again, because Usenet is decentralized, and the files you see are just what's on your specific news server, your news server's admin will generally have things on a rolling cycle where posts get nuked after [x] days. Obviously, higher is better, unless you don't mind checking Usenet daily for your fix.
Monthly Quota - How much can you download per month? Bandwidth isn't free, so servers generally give you a monthly quota. To me, this is the most important aspect, because who cares if you have access to 80 bazillion terabytes of posts and can download them at the fastest speed your hardware can handle if you have to essentially pay per gig? Luckily, pay-per-gig services are few and far between these days.
Price - DURRRRRRRRRR
With that out of the way, here are some commercial news servers:
Power Usenet - $15/month Unlimited, $20/month Unlimited + SSL (First 3 months for both plans are $5 off). Near-perfect completion, 125 days of retention, and they've got a trial that lasts 5 days or 10Gb. They're a Giganews reseller, and really the only difference I can tell between the two services is retention. The NEW SH/SC News Server of Choice if you want unlimited access. Same price as Newshosting, 45 more days of retention, and they can potentially scale up in retention as far as GigaNews will let them.
NewsHosting - $15/month Unlimited w/80 days of retention, $20/month Unlimited w/110 days of retention and SSL, or $10 for 10gb. Near-perfect completion. Retention is now respectable for their SSL accounts, but PowerUsenet still comes out on top. Roadrunner users used to get free access to NewsHosting, but not anymore. Sucks to be you.
GigaNews - $25/month Unlimited, $30/month Unlimited w/SSL, or $13/month for 25Gb. Very good completion, and now an unprecedented200+ days of retention. Their unlimited plans are kinda pricey, but damn, you're getting 200+ days of retention. That's over half a year of binaries. Note that they've just started adding retention, so right now you only get access to ~140 days, but it will eventually top out at 200. Still, this is absolutely unheard of. Keep in mind that many ISPs farm out their Usenet access to Giganews, in the form of very stripped-down 2Gb/mo accounts. There's a chance that this kind of account may qualify you for a discount if you decide you want a larger GigaNews account. Before hopping on the GigaNews SSL Bandwagon, remember to check your newsreader to see that it actually supports SSL. Not all do yet.
UseNetServer.com - $15/month or $40/3-months Unlimited. Good completion, ~110 days of retention. Often goes through periods of lame service, where stuff won't work for any number of different reasons. Now features an .nzb search engine of unknown quality. Pretty much the only upside is that they offer SSL standard.
Newsdemon - $18/mo for 10 connections Unlimited, $23/mo for 20 connections Unlimited with SSL. Newsdemon is a UNS reseller, and i have absolutely no idea why you would pick them over UNS. But hey, if you really want to go with them, you can at least bring their rates down to UNS-levels with 20%-off coupon codes ngreviews20 and retailmenot.
EasyNews - $10 for 20GB or 30 days, whichever comes first. Absolutely incredible completion and a month or more for retention. Features a web-based interface that lets you search for articles and add the files to a zipmanager for easy downloading, but only works on one group at a time. Parses par2 files and outputs to nzb files. If you do their monthly promos, you can get up to 24 gigs over HTTP or 32 gigs over NNTP. Really, EasyNews kinda sucks compared to modern options like Newshosting or Giganews. Plus if you're thinking about posting binaries with EasyNews, think again. They'll gleefully lock and then nuke your account if they receive a DMCA notice pertaining to your postings. Shouldn't be a problem for leeches, but you might want to think about if you want to encourage this kind of behavior.
Megabitz - $14/mo for 5 connections unlimited. They claim ~110 days of retention, but it varies wildly on a per group basis. Some low-volume groups have that much, but anything high volume is going to be closer to 30-40 days. (Don't believe me?) You're better off spending your money elsewhere.
Other Hosts - A bunch of other hosts have crawled out of the woodwork lately. The only one I've still not heard anything specific about is NGroups. They might be great, who knows, but I prefer sticking to the big ones. There are, however, a couple outfits you should probably stay away from, specifically Graboid, Rhever/nzb.com, Usenext and Usenet.com. Stay away from Graboid and Rhever/nzb.com because they're just asking to get sued (and as an extra reason, Graboid is just a closed-source frontend for SABnzbd). Stay away from Usenext because they're sleazy, and stay away from Usenet.com because they're sleazy, retarded, and are now being sued by the RIAA.
An In-Depth Walkthrough
We used to have a lengthy walkthrough here explaining in-depth how to set up and use SABnzbd, but that was only really needed because there was no official SABnzbd documentation. We don't have to do that anymore, because the SABnzbdPlus team is staffed by competent people who like writing documentation. So here's the only three links you'll need to get everything up and running once you've secured a news host, some Newzbin premium credit, and have downloaded the latest build of SABnzbdPlus.
Once you get it all configured, it's just a matter of searching for something at Newzbin and clicking the pretty orange arrows. SABnzbdPlus will gleefully download everything automatically, verify it, repair it, extract it, and clean up the remaining files for you. Anything else is insane and needlessly complicated.