Here's a nice little introductory article on TOR, The Onion Router, a privacy-enhancing technology that helps you to circumvent national, corporate and school firewalls and enhance your anonymity. Originally developed by the US military to help communications get in and out of countries that heavily filter their networks, TOR is free/open software and is maintained by many volunteers around the world, including the Electronic Frontier Foundation.
TOR works by passing your traffic through several (theoretically) unrelated computers all over the Internet, using cryptography to keep the origin, destination, and intermediary steps secret from each computer it passes through.
You can run TOR on your own computers and they'll become part of this array of intermediary hosts all over the net, making your network connection into a tool for privacy and free access to information.
Bill McGonigle, of Lebanon, New Hampshire, decided to become a Tor volunteer when he learned that people in Iran were protesting the results of their June Presidential election. They were using the Internet to organize their meetings. The Iranian government was trying to censor their messages to one another. "I have a soft-spot for people trying to gain liberty for themselves," he wrote in an email, "especially against tyrannical regimes. It became known that they were using Tor to get around the censorship, so at that point I put up a relay....The people I'd like to help are those living under violence-based oppression, most commonly orchestrated by dangerous and corrupt individuals posing as legitimate governments. I'd like to see an end to oppression wherever it exists."