KanecoWith a scene as active as tf2 I can only think of quake but even then, quake live can be considered a different game, and it was released after tf2 iirc. Maybe W: Enemy Territory? But the scene is pretty much dead.
I think the only thing big enough that wasn't CSS since it just came after 1.6 and leeched the scene from it might be DotA, which stayed around from 2003 until Dota 2's release in 2013 (2011 if you count beta). That's a good 10 years I guess, so I guess that's longer than TF2, but it's dead now and if TF2 stays alive a couple more years it'll surpass it.