Best Co-op games v.Personal Computer

Doom 3 does not have supported co-op, which is probably the only thing that would've made it worth playing sadly.

SWAT 4 is pretty cool, but a little slow.

Serious Sam has never really been my game, just not enough to it I suppose.

If you're a flight sim type get your hands on X-Wing Alliance. It's a few years old but you can fly space missions with up to 8 people. (Co-op and H2H).