PC classic game Duke Nukem 3D is free for 48 hours over at GoG.com as part of their Winter promotion. The game was released in 1997 for DOS and became an instant hit for PC gamers. Duke Nukem 3D supports four player co-op via LAN or Internet through the entire campaign. We've seen a recent release on Xbox Live Arcade which bumped the co-op count from 4 to 8, as well. The version on GoG is the Atomic Edition.
So head on over to GoG and grab Duke 3D and while you are there check out the massive Winter sale on games, there's sure to be some classic gems in there like Baldur's Gate,