I was having a look at some of the games I have on Steam and found an amazing game mod called Synergy. It's absolutely brilliant. It's a co-op mmorpg sort of thing based on the HL2 theme. As a team there is problem solving, loads of nasties to fight - all those you would find in HL2. Everything you loved in HL2 is in there, but you're working your way through it as a team.
This is a mod, so the levels are made by the mod'rs. (or whatever you call them) It's not a co-op version of HL2, it's a co-op game using all the features of HL2, it's brilliant. Check out <a href="http://www.synergymod.net/" target="_blank">their webby</a>, and some <a href="http://www.youtube.com/watch?v=nKHkFm-bM7U" target="_blank">vid's on YouTube.</a>