What other games have a cooperative mode like Halo?

My husband recently convinced me to sit down and play Halo with him. Now, I don't hold a candle to his skill, but... we've been enjoying it, and it's been a lot of fun to have something that we can do together.

We tried the 'death match' mutiplayer mode and both agreed that I'm not match for him, and that it wasn't any fun. ...

so... While we have Halo and halo 2 to play through, what after?

I'm looking for some other video games that have a cooperative play mode... something where we can work together as a team...

I'd prefer a game where we're both playing at the same time--as opposed to a game where we have to take turns... but a turn-taking game would be okay.

A game where our only option is to compete against each other is pretty well out.

So... any suggestions?

We have a ps2, and an xbox, and a pair of new PC's. So, preferably, for those gaming systems.

Thanks!