Day of Defeat: Source is a game that came bundled with my purchase of Counter Strike: Source. It is a First-Person Shooter game that pits once again two teams: the Allied forces and Axis powers. There are a variety of different maps that come bundled with this game, exclusively based in the European theater of WWII.
I found thiis game to be a terrific addition to the bundle I bought! This game is absolutely awesome. My 7-year-old PC can run this game 40+ fps on nearly all maps. The online community is excellent. I've found the players to be much more mature, and most servers are family-friendly (for example, no foul language, no pornographic sprays, etc). The gameplay is very realistic. Maps typically have points marked by flags for either the Allied forces or the Axis powers to claim. Once either side captures all the points, the round is ended and a new game begins.
However, there are some cons to the game. The gameplay is unique, as I said, but really, how many war-based FPS games have you seen/played? Perhaps VALVe could have thought of something better. There are a very limited arsenal of weapons. Maps are typically very small.
All in all, Day of Defeat: Source is a pretty good game. I find myself playing this more than most FPSes because of its great community. I'd definitely recommend it to any FPS gamer.