I've been having a lot of fun playing Viet Cong. The realism.. is just great.
My favorite = Tactical Ops. Originally a UT mod, now turned retail. 2.2 was my favorite version, the current is 3.4. Pure MP FPS. This game honed my FPS skills..
Oh and, after playing this for 2 years then giving CS a try.. CS was waaaaay to easy.
ET gets a few hours out of me every now and then too.