PBS
Pbs Learning Media: Relativity and the Cosmos
This illustrated essay from the NOVA Web site introduces the basic concepts of Einstein's General Theory of Relativity and what we know about cosmology as a result.
Science Struck
Science Struck: What Is Gravity and How Does It Work
A very detailed look at gravitational force and the many theories that have been put forward to try to explain it. Includes lots of illustrations.
Wikimedia
Wikipedia: The Black Hole
This site examines the black hole as an object in astrophysics. Delve into this comprehensive resource that covers this concept from its history, to qualitative physics, the reality of black holes, mathematical physics and more.