There is Counter-Strike: Source, there is Day of Defeat: Source, however there was never an incarnation of Team Fortress using the Source engine, until now. After the success of Half-Life, the development team, Valve Software, announced a next-generation multi-player game: Team Fortress 2. However after the release of TFC things went pretty quiet on that front, not until recently we found out what was TF2's real fate.
The new incarnation of the game, with the source engine, has a bit of a cartoon feel, graphics-wise, however the graphics are sharp and very clean. The developers even added a little bit of background story. There are two teams that secretly control all the world's governments: Reliable Excavation & Demolition (RED) and Builders League United (BLU). The characters also have their own background story, each of the nine different classes has its own nationality, and voice. The same 9 classes are back: Sniper, Scout, Spy, Pyro, Soldier, Heavy-Weapons, Engineer, Demolitions and Medic. There are a couple of new gameplay modes, like Payload and Team Arena. There were some changes that didn't go well with some fans, like class-specific grenades - they are gone. However this game is really one of the best multiplayer experiences, whether you are a fan of Team Fortress or not. You can get this game at Steam, Valve's official store.