Modular Combat [official site], a Half-Life 2-based multiplayer RPG-FPS, has been released on Steam. You don’t need to own HL2 to play. Players take the role of Combine, Resistance or Aperture characters and fight to the death against other factions, in free for all deathmatches, or against a variety of Half-Life monstrosities. I’m not sure why Combine Hunters and Manhacks are fighting Combine footsoldiers, but hush Shaun, it doesn’t matter when it’s fun.
There’s more information on the various game modes summed up in this handy trailer, which also goes into a little detail on the various upgrades available to players.