Believe it or not, one of the earliest and best ever is still totally working! Battlefield 1943! It's is a absolute wonderful game. Online only but it always finds a game full of people the second I launch it!
I actually unlocked every achievement in it naturally. (I.e. not trying or even knowing what they are).
Also, I'm a WW II kinda guy, I still get my fix with the cool game mode that is Just two carriers. Us vs. Japan , I can't say enough good things about it.
Another one, But only the Demo is Battle Stations Midway, Its old, no one ever bought it but the demo is still super popular and people play it all the time! DL that one for free and try it.