We (some friends and I) were playing a lot of Counter Strike or even better Battlefield 2 or Battlefield 2142. Left 4 dead also was some fun.
We tried Call of Duty 4, which didn't work out for us, as it doesn't offer enough possibilities to work together as a team.
We are always searching for games that offer that kind of gameplay.
The last I heard from Battlefield - Bad Company 2 it should fit nicely into that description.
Are there others?
As I can only accept one answer I'll take the one that I like most, but will vote for each that could also be an answer to me.