http://allthesky.com/: Digital Images of the Sky
http://physics.weber.edu/carroll/: Modern Astrophysics homepage.
http://www.mpa-garching.mpg.de/galform/press/: A simulation of the entire universe.
http://www.jgiesen.de/astro/astroJS/siderealClock/: Sidereal Time
Stellarium is the most awesome real-time starchart program that I know. Totally free and great to see where things are in your sky right now, tomorrow, or 10,000 years ago. Use it to find stuff in the sky in real time. Download this program NOW if you don’t have it.
Celestia allows you to kind of travel around the universe instead of being confined to a view from Earth. Worth trying, although I didn’t have time to investigate into this one in depth. But it seems to be cool at first glance.
100,000 Stars: Online Google App that allows you to “travel”across the nearby stars. Pretty cool, if you ask me. You can see the (small) size of the Solar System compared to the distance to the closest stars. You can also click on individual stars to obtain information about them. It’s FREE so check it out now!
Cosmic Classroom by Vera Margoniner: simple, easy to understand, and decently explained concepts about astrophysics in youtube.