Act Locally! Extensive, to say the least.
Act Now: A fabulous way to contact local, state and federal representatives from any of the 50 states of the US.
ACTION (Activists' Center for Training In Organizing and Networking): Mostly concentrates on evironmental issues and corporate power reform. Check out Research Tools Online.
Action Coalition
American Student Political Network Sponsored by the Yale Political Union, so it's a little Ivy, but it offers some useful information. A solid resource for campus organizers.
Demonstrator's Site Probably the best site of the lot. Always up-to-date, and provides list of upcoming demonstrations, all over the U.S. Good legal resources.
Electronic Activist An email address directory of congresspeople, state governments, and media entities.
Fight the Right Action Kit By the National Gay and Lesbian Task Force - a serious challenge to the radical religious right.
Human Rights Web "Acts of injustice done/ between the setting and the rising sun/ in history lie like bones,/ each one."
NetActivism "NetActivism is a place to find out about current issues of note, especially ones of critical importance at the moment. The goal here is not just to inform, but ideally to bridge the gap between education and grassroots involvement and action."
Plugged In: After rafts of e-mail asking for advice on starting up satellite programs, this incredible East Palo Alto community computing access center put up a page explaining how they did it, and how you can, too.
Strike Page: It's all about solidarity. Bi-monthly and VERY informative.
Talk Back/Write Back: A Media Action Kit. Start your own campaign to defeat the California "Civil Rights" Initiative - which would end Affirmative Action in the state of California. For more information on the California "Civil Rights" Initiative, go here.
Women's Net Global Faxnet Archive: Global alerts around social issues. Timely and comprehensive.

Alta Vista: An obvious choice. Comprehensive and quick, and the link: and whois: features are, in my opion, indispensable.
Internet Address Finder: Getting more folks and their e-mail adresses in their database everyday. It's often hit or miss, but occasionally very useful.
WWW Servers of the US Federal Government: Handier than it sounds.
The CIA: If you need information, they got it. Hey, maybe they have a file on you!
Senate & House Bills and Records Via Thomas: Legislative Information on the 'Net.
Library of Congress