Scott Rouse has informed us at this thread over at ENWorld that Wizards has set up a Partnerships page which list all publishing companies that have signed the GSL. The list includes links to the companies' website and a short blurb of their offerings.
Some of the big names that are currently in the list are Goodman Games, Mongoose Publishing, EN Publishing and One Bad Egg.
The list is still incomplete and more companies would be added along the way.
For an unofficial list, you could also check out this thread.