Communications for a Sustainable Future-- Home page of progressive mailing lists housed at University of Colorado.
Marcus Green's Leftist Links Archive
Humanities Net -- Home of mailing lists primarily of interest to academics. Excellent archives.
International Global Communications -- Sometimes referred to as "Peacenet", this ISP has lots of very good information archived with excellent search capabilities.
Left Directory (UK) -- another British based guide to progressive internet resources.
Jay Moore's Leftist and Progressive Internet Resources -- Jay's goal is to create something like Yahoo for the radical movement.
Johannes Schneider's webpage -- Directory of Marxism resources on the Internet in German and English.