Gravitational Lensing - A popular site with interactive demonstrations of gravitational lensing. The background material is at a graduate level
Gravitational Waves: An Introduction - This paper presents an elementary introduction to the theory of gravitational waves. This article is meant for students who have had an exposure to general relativity, but results from general relativity have been derived in the appendices.
Gravity Probe B - Gravity Probe B is the relativity gyroscope experiment being developed by NASA and Stanford University to test two extraordinary, unverified predictions of Albert Einstein's general theory of relativity.
Interactive Experiments in Gravity - Try an experiment that illustrates the gravitational attraction between two objects or use a Java applet to understand how orbits work in strongly curved space-time.
NOVA Online/Einstein Revealed - Profile of Albert Einstein, with additional teaching resources, Shockwave demonstrations, and animations of relativity concepts.
Relativity Forum - Forum devoted to discussing the Einstein's special and general theories of relativity.
Ricci - A Mathematica package for doing tensor calculations in differential geometry and general relativity.
Shapiro Radar Bounce Test - A two part overview of the Shapiro radar bounce test of general relativity. (The two parts consist of a section for normal people, and one for nerds)
Southampton GR Explorer - An introduction to Einstein's theory of General Relativity and related topics. These pages include informative text, pictures and movies.
Gravity of Gravity - An experiment at the University of Washington seeks to determine whether the gravitational binding energy of an object generates gravity of its own. (October 26, 1999)