Sven Co-op (HL1)

Sven Co-op is officially coming to Steam for free on the 22nd.  Launched in 1999, it includes the entire Half-Life 1 campaign for play with multiple online players, as well as a bunch of other really crazy stuff in addition to that.

This is free and Valve is allowing anyone to use it, even if you don't actually own HL1.

store.steampowered.com/app/225840/