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