If you keep an eye on the Most Popular New Mashups section of ProgrammableWeb you get a reasonably good sense of which mashups folks think are most interesting. One that quickly rose to the top of the list recently is a Big Huge Labs. Home of "fd's Flickr Toys", it's a great mix of over two dozen toys, games and utilities for your photos.
fd's John Watson has been creatively using the Flickr API since its early days. Hard to beat for innovative use of any photo API.