There's "Red Orchestra". Buy it for $20 on Steam, it's a great FPS!
Maybe it's just me or the type of FPS games I've played for years, but Red Orchestra isn't worth $20. I played Day of Defeat (non Source) for about 4-5 years competitively. For those of you not familiar with it, its sort of like Counter-strike, but capture the flag style and not that realistic. Anyways, the infantry controls on RO aim to be realistic, but they are slow and combersome. I'm all for realism in certain games, but playing RO on infantry maps boggles my mind.
Now their tank sim part of RO is well done. I bought RO when there was a steam sale on it for $5. And for $5, its an excellent tank sim. Though there are only a handful of Tank Only servers left and they're in Germany.