Half-Life is excellent, as well as the mods
Team Fortress Classic and
Counter-Strike, but the action tends to become repetitive. I recently discovered a unique form of
modification known as
Sven Co-op. Developed by Sven Viking, it is the only cooperative mode currently available for Half-Life.
Quite simply, Sven Co-op allows any number of Half-Life players to actually team up against monsters in
(A) the entire single-player game,
(B) a unique set of Sven Co-op maps (all very well done and enjoyable, unless the server running them has problems), or
(C) user-created maps or, preferably, map sets, the best of which currently available (by
unanimous opinion) is Operation Retribution. For more info, the mod itself, and the maps, you may log on to www.svencoop.com.