The "Astronomy Picture of the Day":http://antwrp.gsfc.nasa.gov/apod/ is fairly good source of fairly cool material. Lovely stuff. And what's nice is that it isn't just a photo, wham - bam - thank you m'am. A professional astronomer actually writes a nice little explanation as well. Bonus. And a hint of tomorrows image.
One of my favorite ones was London at Night. It's just so abstract. I mean, what in the hell is putting out that much light in the center of london? The Tower of London? Parliament? Look close and you can pick out Hyde's Park and Regents Park.