Team Fortress 2 is now back to it's roots. The first Team Fortress was a mod for the original Quake and was free also. It had gone through its "Duke Nukem Forever" stage where there was rumours of the development of its sequel and early released gameplay were completely different to the stylised look of the final product.
I believe that it did not share the same success as other similar mod to Valve games such as Counter-strike and Portal. With the complexity of the different classes and different styles of play, it made it really hard for a new player to get started as it is multiplayer only and most players online were quite experienced.
The introduction of Free-to-play to Team Fortress 2 should provide a fresh injection of fresh meat to the slaughter, but should make getting into the game a less tramatic experience.
External Link:
http://www.teamfortress.com/freetoplay/