Time-waster alert: stunning photographs, free screen-savers and wallpapers.
The Small world competition website has some stunning micrographs, photographs taken through a microscope.
I found it easiest to get to the gallery and click my way through them as a slideshow. They also sell acalendar and have archives back to 1977 that include large, high-resolution, screensaver images. Go for it, people!
Hat tip to the Sciencepunk blog.