TripleA is a free turn based strategy game and board game, similar to Risk, or Axis & Allies. Known for its multiplayer on the online lobby, it also allows Play-By-Email and network (LAN) play, as well as single player vs AI or hotseat. It has over 100 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 savegames, encrypted dice gen, low luck, and other options. Recreate World War 2 with the Axis pushing through to Moscow and Japan devouring the Pacific, or recreate Napoleon's march across Europe, or Rome defeating the Carthaginian Empire, or even Sauron conquering Middle Earth, or Zombies taking over America! If you have ever played a game where you push little plastic/virtual pieces around, roll dice, conquer the lands of your enemy, & produce new pieces to conquer with, you will be able to jump right into TripleA!
Twilight Imperium is an attempt to adapt this board game to the TripleA game engine. While it does not follow the board games rules perfectly, it is still quite fun. Must be downloaded separately or through TripleA's in game download-maps feature.