[Foodstamp]

Dwarf Fortress is a much better example (It's still alpha, who is to say it will stay free to play, although I think it will). Battle for Wesnoth would be good too.

But for every game like that, you have dozens of "free" MMORPGs, DOTA clones etc that claim to be free, but then present you with things like item malls or paying to play certain classes etc in order to compete or gain an edge.