Since .onion addresses are de-centralized by design, you'll never access many of them unless you find out about them through word of mouth. However, a selection of them are cataloged in various places, including "The Hidden Wiki" and TORDIR. These two are a good place to start exploring. The content available through these two directories may shock you; the anonymity of the Tor system means some of the most extreme content on the internet is available through here:

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Front page for the TORDIR directory, which is only accessible via Tor

 

Some of the uses for Tor's anonymous servers is perfectly innocent, including websites and blogs from people who simply prefer to remain anonymous and untraceable. An awe inspiring use for these servers has been to facilitate a way for the media to report to the outside world from behind censoring, or vice versa. However, it has also facilitated the distribution of child pornography, the sale of illegal drugs (and instructions for the creation of such drugs), as well as human trafficking. There are even servers where you can hire assassins. And indeed, all of these are protected by the encryption that Tor provides. As it happens, not a single person is known to have been convicted of crimes due to being traced through the Tor network.

That being said, Tor still has weaknesses. Though the IP addresses are concealed, services hosted from a single location can be manipulated to track visits, and though it is secure from tracing, the .onion sites can be hacked just as any other. Last year, the hacktivist group anonymous, in what they called "Operation Darknet" hacked into a child pornography site on Tor and posted the almost 1600 names of it's users.