http://www.jpl.nasa.gov/index.html
Jet Propulsion Labratory
A good place to start for updated Mars information.

http://photojournal.jpl.nasa.gov/targetFamily/Mars
Planetary Photo Journal - Daily updates of Mars images - Anaglyph 3D

http://photojournal.jpl.nasa.gov/index.html
Planetary Photo Journal - All the Planets

http://marsrovers.jpl.nasa.gov/home/index.html
Mars Exploration Rover Mission - Latest News

http://marsrovers.jpl.nasa.gov/gallery/images.html
Mars Exploration Rover Mission - Press Release and Raw Image Link

http://marsrovers.jpl.nasa.gov/gallery/images.html
All the raw image data from Spirit and Opportunity Rovers

http://marsprogram.jpl.nasa.gov/funzone_flash.html
Fun Mars place for kids

http://marsprogram.jpl.nasa.gov
NASA's Mars Exploration Program

http://mars.systemsfirst.com
Great Mars Images and Up to Date News!