Co-operative 2-Player Games

[Matt_H]

You could play Worms in cooperative mode against the CPU or another team of people. What a great game! :-)
Guardian has a 2-Player mode, but I don't know if it's cooperative.
There's also Gloom and Quake (over a network)
The Lemmings 2-Player mode is fun. Though not cooperative, both players do play simultaneously.

That's all I can think of.

[Matt_H]

I always found level 1 of Tricky to be especially good for blowing up Lemmings. :-D