General Relativity (the best gravity theory we have so far):

Relativity: The Special and General Theory - Einstein's 1920 work (translated in English).
Geometry Around Black Holes  - with some math.
Relativity on the World Wide Web - certified by the experts.
The General Theory of Relativity - lecture by R.Egerton..
The Michelson-Morley Experiment - this is really related to the Special Relativity, but every non-relativist theory of gravity must explain it or disprove (or question its results, as this paper by Héctor A. Múnera does).

MOND - really a modification to Newton's laws of motion, but to me (sorry Mr. Milgrom) an example of brute-force curve-fitting

Hypersphere rotation - "Clifford Algebras and Spinors" by Pertti Lounesto. Use "search inside this book" link to search for "rotating ball" -- you'll get to the chapter "6.6 Rotating ball in R4".

for anti-gravity fans: I'm going to add some links here in the near future - it's just hard to find those that make at least a little sense.