Studio 4°C makes gorgeous stuff.
Mushishi is gorgeous in conception and execution.
Some stuff by Gainax is also beautiful and well done: FLCL, for example, though it's not the same level of fantastic awesome as 4°C.
If you like graphic design (as distinct from the more painterly arts), check out SHAFT. ef ~a tale of memories~ is their most recent work of note.
If you like pretty art but don't need high-end animation, check out some of the so-called "relaxan animu", such as Aria or Sketchbook.
I was surprised at the quality of the background art in Elfin Lied and Nagasarete Airantou, but I don't recommend the latter for anything except academic interest.