Canned Tuna is a deathmatch mod for Half-Life. The mod only has five weapons, but each weapon is extremely powerful. The mod also comes with multiple game modes, with each game mode being a variant of deathmatch.
At first glance it may not look like much, but the gameplay sure as hell is extremely fun. It still needs more maps, and the bots have some problems that could use fixing (they just stand in corners sometimes).