I'm not usually a big fan of concept art, but Syd Mead's work is hard to overlook. He basically started the concept art genre after he graduated in the 50's. His portfolio his pretty ridiculous, working on movies like Bladerunner, Aliens, Tron, and Gundam. He's still at it today too, which you can see from his website here. I also found an interview with him and tons of large images over at Miscellaneous Pics here.