As a Linux user good games are few and far between. However I have recently discovered the Freespace 2 open source game. Freespace is a fairly old game that was released open source a few years back. Usually this means that the engine is available for hacking, but the contents of the original game (art, missions, models, etc) are not available. However it seems that for Freespace 2, all these things are available. I’m not 100% sure the content is actually legal, but there are no signs that it is not. Also, as this has been open source, it has also been ported to Linux.
Even cooler, is Beyond the Red Line a total conversion that is implementing a Battlestar Galactica game in the Freespace 2 engine. So far they have just released a demo, but it is pretty good.
Even though I am playing these under linux, they also appear to be available for Windows and OS X.