NASA
Nasa Star Child: Probes to Mars (Level 2)
This site has a description of the different probes to pass by and land on Mars. Glossary, graphics, and printable version are also included.
NASA
Nasa Space Science Data Archive: Photo Gallery: Mars
A huge collection of pictures of Mars, including shots from spacecraft and landers. Extensive coverage of controversial features like the "face on Mars" with comparison photos from Viking and Mars Global Surveyor.