The following links will take you to Web sites which are especially useful, informative, and entertaining.
The Great Shapley-Curtis Debate of 1920 on the Scale of the Universe
Astronomy Picture of the Day Index
Hubble Space Telescope Images
Color Photographs from the Anglo-Australian Observatory
STScI Digitized Sky Survey: The Whole Sky, gif or fits
Falling into a Black Hole (GIF movies)
The Milky Way Collides with the Andromeda Galaxy 3 Gyr from Now (MPEG Movies)
Very Useful but Less Entertaining
Preprint Archive ("astro-ph")
NASA Astrophysics Data System ABSTRACT SERVICE
Prof. Bill Press' Introduction to Astrophysics Course at Harvard U
Astronomical Coordinate Systems
Less Useful but More Entertaining
MIThenge
The Milky Way at 7 Different Wavelengths (Free Poster!)