RTMark.com
If you can't fight 'em, join 'em, then fight 'em more effectively. RTMark (pronounced art mark) became a corporation so they could facilitate acts of corporate sabotage. They spotlight the essential injustice in the corporate entity (that it has all of the rights of the individual, but none of the responsibilities) by using their privilege of limited liability to veil monkeywrenchers -- while stating that one of their ultimate aims is to do away with the concept of limited liability. A brilliant paradox and the most inspiring site I've ever seen.

Adbusters.org
Since I worked on one of the first versions of this site in '95, Adbusters has always used the web for their own subversive purposes: from the "Click Here To Send A Nasty JPEG To Philip Morris" actions of my day to the far more sophisticated multimedia treats on their current site. Still one of the few great magazines that has a site that's more than an online marketing tool.