Important information for Astronomy Lab Students
Bad Astronomy.com: Phil Plait's celebrated website about debunking common misconceptions.
The Cassini-Huygens Homepage featuring up-to-date images, stereo images, movies and information about the spacecraft that's been orbiting Saturn since 2004.
NASA's Mars page with links and information about all of the orbiters and rovers currently at Mars.
The Hubble Space Telescope website complete with a gallery.