I'll try to keep this under five thousand words if I can:
Ascaron Entertainment GmbH. Germany loves its nonviolent builder-type games, and nobody does that genre better than Ascaron. Patrician 3 is the deepest trade-sim/city-builder I've ever played, although Port Royale 2 comes close, and Will Wright himself should take notes about how to do a city builder correctly. They also did Darkstar One (which is either an updated Elite or a single-player EVE Online depending on your perspective), Sacred (yes, it's a shameless Diablo clone but it's a GOOD shameless Diablo clone), and a couple other games I'm sure I'm forgetting. Their new partnership with cdv Entertainment has produced the "share and try it" DRM model in Sacred 2, which may well be the wave of the future. How they talked cdv (which used to use StarForce back in the day) into a less-restrictive DRM model is a mystery, albeit one with a delightful end result.
Paradox Interactive: You could argue that all their games are the same. You'd have a damn good argument. But when you think about it, D&D, GURPS, and Warhammer 40K are all variations on a theme too, and they're all a blast to play. Europa Universalis and its many spawn (Crusader Kings, Victoria, Hearts of Iron...) take the periods of history they cover and tweak the Europa engine to do things that would be out of place in the other games, keeping the history (or the alt-history) accurate and interesting. People have even made mods for Vicky and HoI to give the starting conditions in the Harry Turtledove novels. It's strategy gamer heaven and nobody does it like Paradox. Throw in their Stardock-like approach to DRM and you've got a winner.
Bethesda Softworks: Daggerfall. Morrowind. Oblivion. Fallout 3. No further questions, Your Honor.