What is a .onion site?
.onion, for all intents an purposes, acts like a top level domain (like your .com's, .org's, etc), but is not accessible like a normal webpage. For this reason, it's called a pseudo top level domain. .onions are run through the Tor network (there are at least two text files on that subject, here, by Mr_Scotty and in the second section of the text here, by myself). Not only does this encrypt server-side data, but increases anonymity as well. On your normal web, a website hosting illegal content would be shut down by the country's government that governs the host. But on your Tor Network, not only is it impossible to tell where the site is hosted - it's impossible to take it down, as well.
Okay, that's cool and all, but how do I visit them?
First, download and install the Vidalia bundle from the torproject.org website.
Secondly, download and install Privoxy (latest stable release found here).
Following this, you should edit your Firefox proxy settings to verify the following is set. Go to Tools > Options > Advanced > Network > Settings. Once there, you should be using Manual Proxy Configuration: http proxy: localhost port: 8118
If not, set them to this.
Reboot the computer.
After your restart, not only is your connection automatically filtered through the Tor network, but you should be able to access Tor-only sites.
Here are some links to get you started:
http://kpvz7ki2v5agwt35.onion - The Hidden Wiki - contains all SORTS of info on tor sites
http://xmh57jrzrnw6insl.onion/ - TORCH, a tor search engine
http://eqt5g4fuenphqinx.onion/ - Core .onion, a tor site directory
http://4jbxjjrbakmdcmvb.onion/ - TorMarks, another tor site directory
http://silkroadvb5piz3r.onion/ - The famous Silk Road - a bitcoins-only marketplace for buying drugs and other items.
http://p2uekn2yfvlvpzbu.onion/ - LiberaTor - Weapons and military training information
http://jhiwjjlqpyawmpjx.onion - Tormail for a free and anon email account
http://4eiruntyxxbgfv7o.onion - TorPM for secure Messaging
http://3c3bdbvhb7j6yab2.onion - Totse2.com Forums (this site via Tor Hidden Service!)
http://zw3crggtadila2sg.onion/downornot/ - Down or Not? for Tor Hidden Services
http://3suaolltfj2xjksb.onion/ - TriPh0rce's page. Has a library, a 'cleaned' hidden wiki, as well as other links.
That should get you started. Although it technically is possible to browse .onion sites by using a service called tor2web, I highly discourage it. It does not give protection to client side users (i.e. YOU). And another note - .onion sites will take slightly longer to open than others, but this should not significantly impact your regular browsing speed. If you decide to temporarily turn off Tor, simply disconnect from the Tor network via the Vidalia Control Panel. You can then browse the internet proxy-free.
you can skip all of this and use the Tor Browser. It works to connect to .onion sites without needing to set up privoxy, or any special network configurations. If you are only browsing the internet, and not routing other services through Tor, you can use the Tor Browser, without having to modify your Firefox settings. Cheers!