The only instrumental acts I can think of that haven't been mentioned are Paul Ortiz (Chimp Spanner), David Maxim Micic, Wide Eyes, and The Helix Nebula.

You could also try Walking Across Jupiter, Oh Hiroshima, and If These Trees Could Talk, but I can't remember if they have (guest) vocals or not.

While they do have vocals you could also try Wheel, their album Resident Human is really good.

Good luck in your search!