From Sierra Entertainment:
Sven Co-op is a Co-operative modification for Valve's genre-defining game Half-Life. In this modification, you are not fighting against other players -- they are your allies, and you need to work with them to overcome large numbers of AI-controlled enemies. Co-operative gameplay has been absent from the majority of recently released games, and is sorely missed by fans of the game mode. Sven Co-op fills this gap by providing not only the ability to play the original Half-Life Single-Player game co-operatively, but also a broad range of diverse co-operative scenarios. Though most are set in the Half-Life universe, Sven Co-op's mapping features allow map developers to create their own, unique experiences for players, ranging from realistic military missions to medieval RPG themed adventures.
Sven Co-op is the ultimate Social mod, ideal for LAN parties or for playing with small groups of friends. There is a lack of competitiveness between players; you'll spend your time enjoying yourself rather than trying to beat the top scorer. Participants tend to build strong bonds with others, as they are required to work together and heal each other, this has been shown through the strong community.