I explored the Flickr Mashups -- what fun! I loved the examples: Spell with Flickr, Montager, and Flickr Color Pickr. Also, the Flash app An Interesting 2005 was great. What amazing photos!
I think my favorite is Montager because there are multiple levels to this one. Enter a tag or image and it creates a photo montage that imitates the lead image displayed on the left. Changing the photo or tag creates a new montage with new photos. Interesting effect.