As noted on TechCrunch yesterday, retrievr is a slick photo search application built on top of Flickr's APIs. You use a nice Flash-based paint box area to 'describe by drawing' what you want and it comes back with surprisingly comparable Flickr images. Example to right shows my poorly drawn yellow circle and the very decent results.
There's been a number of interesting apps built on Flickr's API and feeds. Here's 23 examples from the database here: