Editor's Choice: Cryptic Studios
In 2006, Cryptic released two new "Issues" for City of Heroes/Villains. These expansions are free to the players and both added a lot of new content to the two games in one. Then, sensing the competition was close, they managed to move past being sued by Marvel and into a deal with the comic book giant and Microsoft to announce Marvel Universe Online, an MMO that will be available on the Xbox 360. The combination cannot be ignored as the Bay Area developer now has projects for two of the biggest companies in the world.
Honorable Mention: ArenaNet
ArenaNet calls them sequels, but we call Nightfall and Factions expansions. Whatever they are, putting out two products of that scale and success is impressive. They lose out to their NCSoft brothers by a nose.