The Astronomy Picture of the Day (and the archive of old APODs).
The JPL Mars Page.
The JPL Mars Global Surveyor Page.
More Mars (the Odyssey mission).
And More Mars (huge photo archive).
The University of Arizona's Lunar and Planetary Lab.
NASA's Galileo mission site
The SOHO solar imaging mission
The Earth Impact Database (all the craters...)
Extrasolar planet list
Making models of NASA spacecraft. What the heck!
A few years ago we were very lucky to have bright comets to observe, but there is almost always a faint one around for those with small telescopes, see Sky and Telescope magazine's online Comet Page.