The following gaming companies are ones whose works I personally enjoy and highly recommend.
Click on a cover for more info.
Wizards of the Coast
Works: Dungeons & Dragons
Green Ronin Publishing
Works: A Song of Ice and Fire RPG, Dragon Age RPG, Mutants and Masterminds RPG
Works: Pathfinder RPG
Margaret Weis Productions, Ltd.
Works: Battlestar Galactica RPG, Dragonlance, Serenity RPG