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.
Half life wars transforms one game into an entirely different and wonderfully fun gaming experience as no other mod i've come across could.
Great attention to detail, superb execution, decent balance, multiple in-game options, and a responsive & active development community.