Synergy

[Nagash]

this was the original coop mod for HL2 when mods were new for it. It used to be called Tim Coop... obviously a rip off from Sven Coop, but they renamed and made a half decent mod, and looking at their website, it's improved greatly

http://synergymod.net/Home

worth an install for some random coop gaming sometime, plus you can install direct from Steam... go to their website and in the top right corner is a button link to download direct from steam. 300mb... your likely to need HL2 and the various episodes to play them in coop, but it has it's own coop maps