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.
Lambda Wars is just one of those great mods which reinvents the wheel and makes a game into another genre altogether. The effort and genius itself that went into this mod is worthy of a good rating alone.
I can't wait to see where the mod goes next in future updates.