TAPIR, the Caltech Theoretical Astrophysics and Relativity Group.

No self-respecting web page for a course on general relativity at Caltech would be complete without the obligatory link to LIGO.

The Laser Interferometer Space Antenna, a future space mission of NASA.

The gr-qc eprint archive contains recent papers on general relativity and cosmology.

If you're in the mood to kill some time, but want something more interesting than computer solitaire, you might dig the Purdue University Math Problem of the Week.