HotWired have collected some of the best ten mashups for Flickr, a very popular tag based photo sharing site owned by Yahoo! They are as follows(not in any particular order):

Retrievr:
Spell with Flickr:
Tagnautica
Flappr
Captioner!
Fastr
Colr Pickr
FlickrFling
FlickrBall
Flickr Soduku

From HotWired Best Flickr Mashups