Retribution, an authorized expansion pack to StarCraft: Brood War, was published in 1998 by WizardWorks Software, independently from Blizzard Entertainment, and authorized by Blizzard as an expansion pack.
Retribution works within the limits of the original StarCraft game engine, and does not add any explicitly new content to the game, although it does contain over 120 new multiplayer maps, and a large single-player campaign.
Source: Wikipedia, "http://starcraft.wikia.com/wiki/StarCraft:_Retribution," available under the CC-BY-SA License.