Spacetime Wrinkles — The Numerical Relativity site for University of Illinois at Urbana-Champaign's National Center for Supercomputing Applications
space.com — A nice site, updated daily with news about space and space exploration
Hubble's Black Holes — A playful site with interactive animations, good for grades 6 and up
NASA's "Universe" Site — A site highlighting NASA's explorations of the history and origins of our Universe
Stars, Galaxies, and Cosmology — The website for Astronomy 162 at the University of Tennessee at Knoxville; an excellent, in-depth site for all areas of astronomy