There may not be a canonical StarCraft MMO, but this is pretty darn close.
In January of 2011, Ryan Winzen unveiled a StarCraft 2 mod that transformed the popular strategy game into a respectable MMORPG. The mod, titled World of StarCraft, aimed to mirror the gameplay of Blizzard's own World of Warcraft in the StarCraft 2 game engine. Now, a full two years later, the ambitious mod is available for download through Battle.net as a series of custom maps called StarCraft Universe: Chronicles of Fate.
Winzen and his team have been busy turning their mod into something that could easily be mistaken for a standalone game. Players can choose from eight character classes based on Terran and Protoss units from the RTS. Characters will level up, earn new abilities, and even take part in vehicular combat on "mounts." However, due to the fact that the StarCraft 2 engine is unable to support massive persistent world-style servers, only five to ten players will be able to group together in one instance.
Knowing that the traditional scale of MMOs would not be possible to achieve on an RTS engine, the developers of SCU focused on squad-sized raids. The smallish party size is more reminiscent of Diablo than WoW - not that that's a bad thing. The high-action gameplay is backed by a custom-built physics engine created by the mod team. Some professional voice actors have lent their talent to the project, as well as composer David Orr (of Castle Crashers fame), who supplied the game's soundtrack. Not bad for a core development team of five dedicated fans who have never met in person.
The current version of StarCraft Universe: Chronicles of Fate is a beta build with support for only single-player gameplay; Winzen says that multiplayer functionality will be coming once the team is sure that "all the core systems work properly." Despite its drastic changes and polish, it is still a StarCraft 2 mod, so you'll need a copy of the core game if you want to try your hand at the MMO.
Even in beta, StarCraft Universe is an impressive testament to what a few talented people can do with a game engine. The StarCraft 2 editor has already been used to make a handful of other genres, including kart racers and survival horror games, but nothing quite this ambitious. The works of these modders serve as proof that gamers are capable of constructing a lot more than just pylons.