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.
This isn't a new mod. It was first released in 2008, and previously appeared on RPS all the way back in 2010 when Lewis Denby covered mod news. Ah, nostalgia. Development has continued on and off for the past few years, though in recent months the focus has been on getting through Greenlight. There was a slightly panicked project update last year claiming the mod was dead due to changes in Valve's Source resource packages, followed by a second update saying "actually maybe not". It follows that going through Greenlight and getting onto Steam might be part of an initiative to avoid similar problems in future. Modular Combat currently requires ownership of Half-Life 2: Episode 2 in order to install, although developers Team ModCom report they're reaching out to Valve to try and circumvent that requirement - another advantage to getting on Steam, no doubt.
I spent some of my Saturday morning playing a little Modular Combat on a German server, and it was good! I was very much in need of some fast-paced shooting after a long week and it delivered, particularly running around the Doom E1M1 map blasting monsters alongside fifteen other players. I didn't earn enough levels to tinker with the various character modules and upgrades much, although I did treat myself to the ability to spawn friendly turrets. Can't go wrong with a friendly turret; Portal 2 taught me that.
The project's ModDB page lists the sponsored servers the game is running from; there's also talk on there regarding dedicated servers.