Sven Co-op

Sven Co-op is a modification of Half-Life made in 1999, implementing a co-op/multiplayer element within the Half-Life campaign and its expansions, as well as allowing others to make maps for others to play on.

Ever since its release, Sven Co-op's development has been on and off, and the campaign of Afraid of Monsters, another Half-Life mod, remarked for its grungy horror aspects, was ported over to Sven Co-op by mapper and coder Cocyx the Gay Skeleton in 2008.