Currently listening to Yuki Kajiura's instrumentals at the moment to try and unsucessfully lull myself to sleep. She creates such a diverse range of music and you can't help but get sucked into them, even as short as they are.
Some of my more favorite tracks of hers are:
Fairyland:
Click to view
Hear Our Prayer:
Click to view
Labyrinth:
Click to view
Bijo to Yajuu:
Click to view
Song of Storm and Fire (and some of you guys may have heard this one, as it's rather popular to use in AMV's):
Click to view