I quite like the following (not going to break copyright)..
http://www.ericjhellergallery.com/in...e=image;iid=66
http://www.ericjhellergallery.com/in...e=image;iid=70
http://www.ericjhellergallery.com/im...xponential.jpg
I particularly like photos of the natural world, geometric shapes and sciencey stuff (like Dark Side of the Moon's cover).