there's also Battlefield 2 which is still pretty popular and Battlefield 2142, kinda sci-fi (not completely :P), which I would say is fairly popular
however I think if you are low level it's a bit harder than COD4, and you don't level up too easily as in COD4 (at least *I* didn't), as you have to pick a class and really play as that class to improve.
the words above were backed by NUCLEAR WEAPONS!







