Need a good Co-op/LAN game

So my roommate is getting into PC gaming and we've discovered the joy of LAN-ing and co-op.

Problem is most of the games I own are single-player only, focuses on large-scale multiplayer, or he's not interested.

So, I'd like some suggestions for some great PC games for good, wholesome LAN/Co-op fun.

We've mostly played RTS games like Command and Conquer 3 and Unreal Tournament 3, but we want to have something new, preferabily in the shooter genre.

And since we're poor/cheap college students, I'm looking for something that I can install on two machines without having to worry about authentication. Any games native to or requiring Steam is out. (Though, I wish he'd pay the $20 for counter-strike source).

Thanks in advance!

Hmm

Good suggestions.

I loved freelancer, but my friend is a bit of an anachronist (all he watches are movies from before the 1960's), so sci-fi doesn't really suit his tastes.

No One Lives Forever 2 might be a good choice.

Any other suggestions?

(He has an xbox and we've regularly played Call of Duty 4 and Rainbow Six: Vegas 1 & 2 a lot, but they're kind of old for us now; we need something new =P)