Here's a cute one, called FlickIt which generates thumbnail images based on a word. In this example, I set the image size to 85 and the query (or subject) to "book":
Images from FlickIt!
FlickIt "is the simple yet extremely useful web service for dynamically generating thumbnails of anything."
The images are somewhat random and change with each screen refresh, lending a certain air of unpredictability but a nice feeling of dynamic content.
Another fun mashup is depictr which combines photos from FlickR with song lyrics.
