Empires, the award winning Half-Life 2 mod, combines the best of the first person shooter and real-time strategy genres. Fight as either the Northern Faction, remnants of the fallen nation of Jekotia, or as the imperial Brenodi Empire. Empires features:
Amazing Mod, innovates in an absurd amount of categories and pushes the Source Engine to its limits.
Specially fun for fans of the Battlefield Series, due to the high amounts vehicle types and infantry combat and variation of terrain.
This mod adds classic RTS elements of base building, setting one player as the "Commander" of the team. Other players are considered "units". There's Resource gathering (don't worry, they're all automatic, so there's no manual labor for this one) and many classical research functions that make units better overral. There are also factories that create vehicles for players to use. The only problem here is not with the mod, but the highly unpredictable behavior from the players, that tend not to follow orders.
The game can be easily griefed. Though, when there's cooperation, the game turns out to be absurdly entertaining.