TripleA is a free turn based strategy game and board game engine. Has single player vs ai/hot-seat, multiplayer through Play By Email, network play, and a hosted online lobby. Comes with 4 games, and over 100 more games and maps that can be downloaded from the user community. It is open source, needing only Java 5 to run (can run on Windows, Mac, Linux, etc). Supports features such as savegames, encrypted dice gen, low luck, and many other options.
And with further tweaking of the engine, and a good deal of abstraction of things, I was able to run a full version of Chinese Go (also known as Igo in Japanese).