The pinnacle of Half-Life mods, Sven Coop is a multiplayer coop FPS based on the GoldSource engine. It features the entire Half Life Campaign which you can play FOR FREE, and a whole host of other user-created content.
This game is very fun indeed. There is always an active playerbase, and many of the maps have a very unique variety to them. Highly recommended to get a few friends along, and play a campaign. Besides the Half-Life campaign, do try out They Hunger and Toonrun. 9/10.