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, the stuff that isn't just 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 an indexer, a browser plugin, 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. Sadly, development of hellanzb has more or less dried up.
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. The world of Usenet Indexers has sort of exploded recently, and there's no real one-stop-shop anymore like Newzbin used to be. The developers of SABnzbd maintain an up-to-date list of decent nzb sources here.
Usenet Servers
When it comes to hosts there are three main things you should care about: Price, Completion and Retention (and probably in that order). Price is an obvious concern, but nearly all plans are in the $10-$15 range, so it's pretty much a wash. Retention is a measure of how many days of content a host keeps on hand. It's also roughly the same wherever you go, and it's stupidly high these days. You'll generally see it somewhere around 400-500 days, with most hosts growing to some great unknown capacity. Completion is the big deciding factor right now, as it's a measure of how reliable a host's news feed is. A host with shit completion will result in missing parts and you not being able to finish downloads. Only solution to this is to get a better host.
NOTE: There are only really a couple major news feeds out there. Giganews (Sold as Giganews, Supernews, PowerUsenet, and several others), Highwinds (Sold as UsenetServer, Newshosting, Easynews, and many others), Astraweb and UsenetNow/BlockNews. Nearly everyone else out there is a reseller. Why? Because running a news feed is hard. Why does this matter? Well, it means that when it comes to completion, if a file is - for instance - missing on one Highwinds reseller, it will almost assuredly be missing on another Highwinds reseller. In general, switch between resellers of the same service for price only and switch between resellers for price, completion or both.
With that said, here's the latest recommendations:
Do you download less than 80Gb/mo?
You want a block plan. Best deal for these is Blocknews, a UsenetNow reseller, which has a 200Gb for $25 plan. If you download less than 80Gb/mo it comes out cheaper than $10 per month, and the blocks won't expire. This is a pretty decent way of testing if you really want to do this whole Usenet thing, especially since if you're [i]really[/i] unsure you can pull it down as far as 5Gb for $2.75, and incredibly low barrier for entry.
They also have even cheaper plans: 500Gb for $57 (11.4c/Gb) and 1Tb for $90 (9c/Gb). These bring the "should you use a block plan?" threshold up to ~87 and ~90 Gb respectively, but I can't really endorse them. For it to be economical, the plan should last you 10-12 months, and that's a long ass time in the Usenet industry. A lot of stuff can change in a year, so it's rarely a good idea to pay for that far into the future.
Do you download more than that, but still want it as cheap as humanly possible?
You'll be looking at a budget unlimited plan, then. The absolute best price per specs deal right now is UsenetServer's $10/mo special?. They're a Highwinds reseller, you get SSL, 20 connections, and ~500 days of retention. If you'd prefer access to the Giganews feed, you'll want to go with Supernews $9.99 special which includes SSL, 30 connections, 400 days of retention. Personally, I prefer the Giganews feed over Highwinds since some people have recently been reporting completion problems with both Highwinds and Astraweb, and I have yet to find something broken on Giganews. But on specs alone UNS' plan is the better of the two.
Astraweb also has a super-cheap special for $11/mo but I can't recommend them at all anymore due to the constant stream of complaints of missing articles. They seem to be getting better, but their lack of a public answer to what the hell is going on doesn't instill much confidence. Plus both the UNS and Supernews plans are cheaper, so why bother?
Are you loaded, or want the absolute most retention ever? Giganews is what you want, then. Their plans have the highest retention in the industry, typically a couple weeks ahead of Highwinds / Astraweb, with excellent completion, but their plans are triple the price: $25/mo and $30/mo, without and with SSL, respectively. For a limited time, however, they're running a special where you get unlimited transfers + SSL for just $20/month (if you get in on this deal, that's the permanent price - not a teaser rate). Only real benefit you get out of going with Giganews Proper over one of their resellers like Supernews is retention, but if you want the extra 250+ days of retention, an extra $10/month makes this one of the best deals in the industry right now.
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 SABnzbd 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 SABnzbd.
Once you get it all configured, it's just a matter of searching for something at your indexer of choice and clicking the pretty orange arrows. SABnzbd 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.