If you are into WWII Submarine games and you're running a Macintosh you're in luck. A game I have played on Linux a few times is now ported to OS X (PPC and Intel). The game is called Danger Deep. The Mac port is here for more information and  Windows\ Linux downloads go here .

One game that I always install on every platform I own is PySOL! This has been a favorite of mine for years and runs on Windows, Linux, and OS X. Unfortunately it is no longer under development, but some fans of the original have picked up development on a fork and it kicks major ass! Not only can you play the popular Klondike, Freecell, and many others, but now you can play Mahjongg in many flavors also! Get a copy of this awesome game system here , which is written in Python!   

Dreamchess is one that I have been waiting on to mature and it's pretty damned good at this point. If you're interested in chess and want a nice 3-D board give it a try. The chess engine is pretty damned good. Dreamchess is here .

These are the few I wanted to mention, but I have a list of great open and free games here if you want more. Let me suggest Neverball, Funguloids, Nimuh, and Wormux. For a classic FooBillard is a must too.