Alien Swarm: Reactive Drop

Alien Swarm: Reactive Drop is a free-to-play top-down tactical co-op expansion on the Alien swarm game and Source SDK developed by a group of designers at Valve who were hired from the MOD community. Reactive drop no longer grants players access to the Source SDK, however it does offer expanded co-op, with up to 8 players, Steam Workshop support, four PvP modes, a single player mode, and new aliens, weapons, achievements and more.

The game sends players on a bug hunt and is a blend of co-op and squad level gameplay. Players team up with friends to form a squad made of IAF Marine classes and use a selection of unlockable weapons featuring a wide variety of loadout configurations to battle aliens in an offworld colony.

Reactive Drop is spectator friendly and features leaderboards as well as the ability for spectators to see hacking minigames and mouse movements in real time. It is available via Steam.