Modular Combat is a role-playing shooter based in the Half-Life universe. There are over 50 modules that can be upgraded at any level, in any combination. Modules give combatants amazing abilities such as flying, teleporting, spawning minions, and shooting flechettes or energy balls.
Update: If you are updating from 2.0.5 or 2.0.5B, this is the version you want to download. Extract it to your "steam/steamapps/sourcemods/" directory. Navigate to that folder and make sure that there is a "modularcombat" folder present and then restart Steam. Modular Combat requires the Orange Box (HL2, HL2:Ep1, HL2:Ep2, HL2DM)
Modular Combat 2.0.6, released July 21, 2012.