Off the top of my head and in no particular order...



Battle for Wesnoth: A turn-based strategy game set in a fantasy realm. What I liked most about it is that units gained experience during battle and level up to more powerful units during the game.



The Babylon Project: A standalone conversion of Freespace 2 (You don't need FS2 to play it). Space flight simulator set in the Babylon 5 universe.



Moonbase Alpha: An actual NASA simulator I stumbled across one night. It was a hoot for awhile.

TF2 is definetly up there for me.

Barring the huge number of free flash games online (For better or for worse)



One of the best FTP games i have played was BatleForge. Not too bad for a RTS game, and if your willing to fork over spme dosh (Not the intent of the thread im sure) it gets soo much better.

Galaxy Conflict German space strategy browsergame. I just like a lot that you have a techtree and research hulls and modules, so you actually end up modeling the ships innards to your likings.

Also there are three different classes with different strategic purposes, also depending on the race you choose.

Sadly the community is dwindling, but alas a solely german localized game has not much perspective.