Hubble Heritage Project
Spectacular new views from the Hubble Telescope are updated monthly.
National Space Science Data Center
NSSDC provides spectacular images of the Planets, Asteroids, Comets, Solar System, Sun, Nebulae, Galaxies, and Globular Clusters from a variety of sources. Enjoy.
Earth and Sky
National Public Radio's Block and Bird have a website that has lots of useful info. One particularly nice feature is called "Tonight's Sky" wherein they point out a different feature that can be observed around 9 PM.
NASA's site with all of the info you will ever need for tracking/observing satellites and the Shuttle.