Lambda Wars (formerly Half-Life 2: Wars) brings the struggle between the human resistance and their Combine overlords to a multiplayer real-time strategy setting. The Lambda Wars beta only requires Alien Swarm installed to play. It features 3 multiplayer game modes: Sandbox, Annihilation and Overrun, showcasing the Rebels and Combine playable factions and a full-fledged RTS interface and HUD. There is also a singleplayer tutorial and a handful of singleplayer missions.
That's how they used to have fun at Woodland rebel settlement before the war tide reached it (hlw_woodland map by JJ, remake of hlw_camp).
Thanks to Sandbox unit control bug that visually takes away the weapon, humans can play soccer unarmed)