Old hat, but every once in a while I re-notice Abstract Strategy Games, which has a really large list of games, including rules to many of them. Which makes the GIPF guys all the more impressive for coming out with six unique games that are actually good.
Lots of great stuff for the abstract math Geeks at Combinational Game Theory, including papers by John Conway, Richard Guy, and others who literally wrote the books on math and games. A treasure trove of fun.
And continuing in an abstract theme, the entire bible as done in Lego, at The Brick Testament.