Here’s a bit of nostalgia for you: Sven Co-op [official site] is now on Steam. The seventeen-year-old Half-Life mod is available as a free standalone game, and—thanks to a fair bit of generosity on Valve’s part—includes all of the Half-Life campaign maps. For free.
That means you can head to Steam, download the game, and start playing co-op Half-Life right now. Again, for free. No need for you or anyone you’re playing with to own Half-Life proper.