Sven Co-op (usually abbreviated as SC) is a cooperative game multiplayer modification for the first-person shooter computer game Half-Life, where players must work together against AI controlled enemies (NPCs), solve puzzles and complete other objectives (usually specified in a MOTD) as a team.
The game was originally created and released by Daniel Sven Viking Fearon as a small mod with two single player maps and the game configured to allow NPCs in the game (by changing a CVAR to allow them to exist in the multiplayer game mode). Sven Coop was released on January 19, 1999 - before Valves Team Fortress Classic - and is therefore the oldest Half-Life mod in existence that is still in development. It has since grown to become a full blown mod, with a full team, and one of the most played games on Steam as of September 18, 2006, excluding the official games by Valve.
You will need HL1 (Half Life 1) purchased on your Steam account and installed, or find a non-Steam HL1 somewhere. HL1 is cheap to buy, and most of the Sven Coop servers are Steam based. This is a fun, free, legal mod.
Enjoy, have fun, and seed please!