One of the artists we have discussed a little in the Concept Art course was Dermot Power. He has done work for a range of projects including several Harry Potter films, Star Wars EpII and others. His sketches quickly flesh out ideas and show a great level of detail that help to ground the concept in the world for which it was destined.
I like his headless horsemen and prop concept work for the HP films - here are a few samples, check his site for much much more.
[concept art by Dermot Power from the Harry Potter films via his website]