Garry’s Mod, which basically allows you to interact with the Source engine to muck around with HL2 (and other game objects), was/is great. From machinima to Concerned and everything in between. Seems it also works with Team Fortress 2, as can be witnessed above, with proof that there was cake in the Orange Box. Somewhere. It just got misplaced.
Guess What Works With Garry’s Mod? TEAM FORTRESS 2! [Borys @ NeoGAF]