DITTO on Shostakovich. Also don't forget Debussy, Rachmaninoff, Mozart's Requiem, and Handel's Messiah. Staples of my diet.
*Ulver - Porn Pieces or the Scars of Cold Kises
*Blue Foundation - ANYTHING, but especially Sweep. 11 minute of epic bliss.
*Anything Kattoo has released (fellow Beefcake fans, do check it)
*The Flashbulb - Soundtrack to a Vacant Life
*Benn Jordan - Pale Blue Dot (it's ambient, it's both dark and relaxing, I think a lot of people here will enjoy this)
*Lung - Afterlife (19 year old dubstep producer releases this song and I'm blown away. This is an amazing track. Even if you Hate Dubstep -tm- I think this will prove you wrong. It's not in that vein at all. If you have the Side-Line app on your phone, it's streamable on there. Otherwise the EP is available on iTunes for a measly US$1.98)
Seconding PJ Harvey
Seconding Radiohead - Talk about a "mainstream" band that can be as experimental as they are and still be, well, "mainstream".
Nick Cave & Warren Ellis' - The Proposition OST
Atticus Ross - The Book of Eli OST
O. Children - "O. Children"
That's all I have for now. Quite a variety there.