I'd really like to compile a list for the GSL, but right now, here's the list per Paizo as of today for gaming companies that have signed on to produce material under the Pathfinder license. Some are known, some are...not. Feel free to add updates as they come along below. (There are also some individuals without websites who have signed on, but I have omitted those for now).
500 Penny Design
Dane of War
Expeditious Retreat Press (Hoorary!)
Fat Dragon Games (also signed on for the GSL)
Louis Porter Jr. Design
Necromancer Games (also signed on for the GSL)
Open Design (Hooray again!)
Reality Four Studios
Scrying Eye Games
Sean K. Reynolds Games
Scorced Urf' Studios
Legends of the Shining Jewel
White Plume Games
I also believe (but have not yet confirmed) Adamant Entertainment will be producing product for Pathfinder. Hopefully, we can get a confirmation on that sometime soon. As an update, we've also heard from Tricky Owlbear Publishing and Reality Deviant Publications, and they're both a yes.
UPDATE: 0one Games has also confirmed!