Even thought it is free now, it's been sold the traditional way for a long time so I don't think it quite counts as a freemium game. Valve's already reaped most of the sales profits it's going to get from TF2.
Except for the TF2 in-game store. Valve does a pretty brisk business in hats, weapons and gadgets, so much business that they're able to offer new content regularly for a four-year old game.
