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, but you must own HL2: Episode 2 on your Steam account. 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.
Demonstration of the capture the flag gamemode.
In this gamemode you must capture the flag to win the game and hold it for 2 minutes (the stick without a flag in this case).