Really fun game that is pretty much a mix between Team Fortress 2 and League of Legends (or DotA, Heroes of Newearth for those not familiar with league of legends). Essentially, it is a team based multiplayer first person shooter you pick a class at the beginning of each round and attempt to break into and destroy the other teams base. Throughout the course of each match you can escort waves of computer controlled robots in to attack, as well as gain money which you can use to upgrade your skills, build/upgrade turrets to defend your base, as well as purchase squads of additional or more powerful robots to assist you. You also gain some persistent money for each win, which can be used to create custom classes and purchase certain other unlockables.
The game is available on Steam for only $15 (think there is a downloadable xbox version too, but not sure), and is quite under-rated if you ask me. I definitely recommend it to anyone who wants something a bit unique, especially for such a low price.