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.
Ah yes! This mod was, to my surprise, a very fun experience when I first tried it back in 2008. Even if it was it's own mod it still is a very insane one to try out. The game is an MMO/RPG/FPS-hybrid, so with that said there is nothing at all like Modular Combat! Download it here or on steam, and just give it a go with friends! You will not be disappointed!