Link: www.svencoop.com (opens in new tab)
The daddy of all co-op mods is showing its age. It's a game in itself, enabling you to trawl through the whole singleplayer story of Half-Life with friends, but also providing access to reams of custom maps and challenges set by the community. It even has its own mods. Yes, it's that rare thing, a mod with mods. It's the slickest, most diverse mod in this collection, with straight shoot fests as well is incredible puzzle maps to explore with friends, all downloadable within the in-game server.