Are There Any Good Books on Relativity Theory? - The Physics FAQ's guide to relativity books; by Chris Hillman (with contributions by Nathan Urban). An extensive annotated list of semi-popular books, textbooks and background reading.
Beyond Einstein - Description of NASA's Beyond Einstein missions set to explore various consequences of general relativity - and possibly, as the name indicate, go beyond what Einstein's theory predicts. Includes educational resources.
Cambridge Relativity Public Home Page - Various pages with non-technical texts about cosmology, black holes, cosmic strings, inflation, quantum cosmology, and string theory, written by members of the Relativity Group at Cambridge University.
Einstein Online - Information about Einstein's theories of special and general relativity and their applications; site is hosted by the Max Planck Institute for Gravitational Physics. Includes a simple introduction, a collection of articles ("Spotlights on relativity"), and a relativistic dictionary.
An Essay on General Relativity - Overview of the basic ideas and principal applications of general relativity. Written by John L. Safko for students in the self-paced astronomy courses at the University of South Carolina in 1997.
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.
Interactive Geometric Database - Interactive database developed by Mustapha Ishak and Kayll Lake and hosted by Queen's University; allows the user to search for specific solutions of Einstein's field equations (that is, for specific model universes of general relativity).
Introduction to General Relativity - By Gerard 't Hooft (Utrecht University); based on lectures held in 2002; a thorough introduction starting with accelerated frames and including topics such as black holes, the basics of cosmology, and gravitational radiation.
The Light Cone: An Illuminating Introduction to Relativity - Introduces the basic principles of relativity, the physics of special relativity, and some basic principles of general relativity. Contains a great number of helpful images and animations; site created by Rob Salgado (Syracuse University).
NOVA Online/Einstein Revealed - Profile of Albert Einstein, with additional teaching resources, Shockwave demonstrations, and animations of relativity concepts.
QMW Hyperspace - A set of hypertext based services for general relativity research provided by the QMW Relativity group.
Relativity - Provides information on the history, experiments and paradoxes of relativity.
Relativity bookmarks - Large collection of relativity-related links by Rob Salgado (Syracuse University). Warning: page hasn't been updated since 2000, and quite a number of links are now broken.
Relativity: List of Important Equations - Page scans from the Handbook of Space Astronomy and Astrophysics giving equations and formulas for special relativity and relativistic cosmology. Includes bibliography.
Selected References in Relativistic Cosmology and General Relativity - Extensive list of classic and modern research papers in cosmology and general relativity. Although the author warns that his selection is subjective, the result is a highly useful list of references; for many papers, there are links to online versions on the arXiv.org e-print server. By Henk van Elst (Queen Mary, University of London).
Wikipedia: General Relativity Resources - Annotated list of reading material about general relativity: popular books, textbooks, books on specific topics, web courses, and websites.
General Relativity Simulation Contest - Attempt at a content aimed at the Internet community to write a simulation program, based on a simple algorithm incorporating the laws of general relativity, that can realistically simulate the behavior of black holes, binary stars, and the twin paradox. (December 28, 2005)
Numerical Relativity: Status and Prospects - Review article by Luis Lehner about the foundations of numerical relativity and recent progress in the field; particular attention is paid to simulations of black hole spacetimes. (February 19, 2002)
Subtle is the Gravity - Review of the motivations for and basic principles of general relativity, aimed at a general audience. Written by Naresh Dadhich (IUCAA Pune), based on a lecture delivered in 2001. (February 3, 2001)