Funcom. They made what is probably the best pair of adventure games outside the Myst series, and have an adventure MMO coming up (along with a sequel to the aforementioned adventure games) that look set to completely blow the competition out of the water. And yet so few have heard of them. Despite the fact that they developed a series that has as a main character the most realistic and best representation of women in ANY game, EVER. Not just an opinion here, but something I'm even willing to claim as fact.
And since I mentioned Myst as well, Cyan are pretty unrecognised too, and yet deserve plenty of praise. They made the Myst series, and did a brilliant job at creating a series with a unique premise, engaging characters, an incredibly detailed and well-written storyline, and some very interesting, unique, intriguing and challenging puzzles throughout...