One of the FOSS projects I really hold dear (mainly because they chose to include a few models made by myself :p ) is UFO:AI.
For those totally out of the loop: it's a turn based strategy game based on the classic game X-Com utilizing an improved Quake2 engine.
Recently they had a new release, version 2.4, but the changes upcoming are even cooler (new soldier models and a greatly improved GUI):
More cool screens of those upcoming changes can be found here and here. And here you can find some more as part of a slightly older, but still interesting interview with the developers.
Besides these development news, there seems to have been an informal competition of who can do the nicer Let's Play videos of UFO:AI ;)
Click here for Part 2 and Part 3, or check out this series of Let's Play videos as the alternative: 1, 2, 3, 4, 5 (Part 1 starts out with an outdated version though).
So if you are hooked now by this awesome game, you might as well check out some alternatives: OpenXcom and XenoWar. Neither of which has as much polish and nice graphics as UFO:AI however.