®ark
http://www.rtmark.com
With the tongue-in-cheek title "Corporate Consulting for the 21st Century", this site offers documentation of the wide range of creative subversion this anonymous network of pranksters have engaged in. They're probably best known for swapping the voice boxes in Barbie and GI Joe dolls, but they've done much, much more, including hilarious spoof web sites of George W. Bush's election site and the World Trade Organization site.
Adbusters
http://www.adbusters.org/
Definitely the highest-profile organization and/or publication associated with this topic, and still a personal favourite of mine. Informative, entertaining, and exceedingly thought-provoking.
Media Filter
http://MediaFilter.org/
This "free media" site features a variety of issues and projects, from the Internet Democracy Project to Covert Action Quarterly, the Shadow underground newspaper, and more.
Modern Television by Phil Patiris
http://www.moderntv.com/
An online television parody, which like all good culturejammers seems to be eternally getting sued by someone, in this case, the CBS television network. Their "Media Deconstruction and Fair Use" statement is a must-read.
Noam Chomsky Political Texts Online
http://www.geocities.com/WallStreet/1928/
A site attempting to fill the void left by the disappearance of the Noam Chomsky Archives site. Just a few articles so far, with pointers to more elsewhere.
Sniggle.net: the Culture Jammer's Encyclopedia
http://www.sniggle.net/
Originally entitled "Trolls, Hoaxes, Culture Jamming, Poetic Terrorism, Media Hacks, Frauds, Impostors, Spoofs, Counterfeits, Fakes, Pranks, Scams, Extraordinary Popular Delusions and the Madness of Crowds," this site has grown from a single page on GeoCities into a massive storehouse of information on the theory and practice of culture jamming, pranks and hoaxes of all varieties. Highly recommended.