HλLF-LIFE²: Deathmatch

Half-Life 2: Deathmatch is a multiplayer first-person shooter video game developed by Valve Corporation. Released on Steam on November 30, 2004, it uses many of the assets from Half-Life 2, and the same Source engine. It features unique levels, optimized for multiplayer arena play, and a few new weapons. Also included are portions of the game's source code, which were the basis for many early Source-based multiplayer modifications. The game is the successor to the popular multiplayer component of the original Half-Life, but is offered as a separate product from Half-Life 2. Deathmatch, like Half-Life multiplayer, does not develop any part of the plot or story of the Half-Life series.

Source: Wikipedia, "Half-Life 2: Deathmatch", available under the CC-BY-SA License.