Mars Pathfinder: the mission that deployed the Sojourner Rover and returned 3D color images of the Martian surface (JPL)
Mars Global Surveyor: currently sending back high resolution photos from the Mars Orbiter Camera (MOC) and detailed topography from the Mars Orbiter Laser Altimeter (MOLA) (JPL)
2001 Mars Odyssey: 2001 Mars Odyssey is part of NASA's Mars Exploration Program, a long-term effort of robotic exploration of the red planet.
Mars Exploration Rovers: Two rovers that are investigating the soil and rocks on opposite sides of Mars: Gusev Crater and Meridiani Planum.
Other Mars Info
Volcano World: Volcanism on Mars (University of North Dakota)
Mars Volcanism (HAS)
The Face on Mars: Geological fluke or sign of intelligent life? Dig around this site and decide for yourself.
Mars, Water, and Ice: Animations showing the formation of various ice-related landforms.
The Case for Mars: Home page for the international conference for the exploration and settlement of Mars.
Martiansoil.com: Daily blog of Mars news