Lambda Wars started out as a mod for Half-Life 2, but later became a standalone project to create an RTS game set in the Half-Life universe. The project has been in progress since 2007 and now, some 13 years later, the game is finally out of beta and is playable for free on Steam.
Lambda Wars has a single player, co-op and multiplayer mode. The so-called asymmetrical gameplay is also unique. The human rebels cannot compete with the military superiority of the Combine, which means that both parties play very differently.