Game: Half-Life 2
Both the paid-for Garry's Mod and the free 9.0.4 version enable you to play the original Half-Life 2 maps on servers - which is possibly a revelation to some people as it's not widely advertised. All you need to do is start a LAN server and select one of the HL2 maps from the map list and you're set. And because you're playing via GMOD, that means instant, boil-in-the-bag co-op, communally battling all of Valve's carefully placed beasties, and with the bonus that you're able to use all of GMOD's resources to set devious traps. The only problem is that the levels don't string together as they do in the full game, but all you have to do is load the next level manually. If you fancy a more tuned experience, there are other, more specialised, mods (Follow Freeman and Synergy), but GMOD is all you really need.