Danish Mars Project - Magnetic properties experiments on Mars - On Pathfinder and future missions
Exploring Mars - Mars exploration, from Pathfinder to manned landing - the very best of what there is has landed here in concise style.
Mars Direct
Mars Republic - A virtual Mars community.
Mars Society of Caltech/JPL
Martian Soil - Daily blog covering Mars related news, both in the field of scientific exploration and popular culture.
Planet Mars - Information and links on Planet Mars, Mars Exploration and Mars Missions, presented by SEDS
Planet Mars Home Page - Worldwide Non-Government Mars Settlement Project
Red Colony.com - Learn about Mars colonization and terraforming with the latest news, images, downloads, newsletter, chat and more.
Red Planet Trek - Site features pictures of Mars taken with MGS by an amateur, includes almost 100 links to Mars info.

American Museum of Natural History - Welcome to Mars
Arctic Mars
Association of Mars Explorers
Beagle 2
Case for Mars
Center for Mars Exploration
Cosmic Ancestry
Daily Martian Weather Report
Imager for Mars Pathfinder
Mars Academy
Mars Atmospheric and Geologic Imaging
Marsbugs
Mars Consulate
Mars graphics
Mars Microphone
Mars Mission Research Center
Mars Network
Mars News
Mars Prize
Mars Project
Mars renders made with MOLA data
Mars Society Canada
Mars Society Germany
Mars Society Japan
Mars Society New England
Mars Society Puget Sound
Mars Society Russia
Mars Society Spain
Mars Society Toronto
Mars Watch
Mars, the Second Earth
MarsDaily.com
Mission Mars
Missions to Mars
Planet Mars
The Mars Society Australia
Think Mars
West to Mars