Here's a trailer, though it's a bit hard to tell what's new and what's not. The standalone Steam release launches alongside Sven Co-op version 5.0-yes, a seventeen-year-old mod is still in active development. You can also play through Blue Shift, Uplink, and Opposing Force, were the inclination to take you. Also: improved enemy AI, additional weapons, a revive ability, new monsters, a new physics engine, high-definition models, and an optional third-person camera. The main difference with Sven Co-op is that Half-Life's been rebalanced for two players, adding more enemies and some two-person puzzles. No need for you or anyone you're playing with to own Half-Life proper.

That means you can head to Steam, download the game, and start playing co-op Half-Life right now. 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. Here's a bit of nostalgia for you: Sven Co-op is now on Steam.