A great collection of the various Flickr applications (even I was surprised at some of them).
It's a great evolution of the original photo sharing application.
Whilst this may be of general interest the reason for posting this is to highlight....(click on the screwdriver section now)....if you have an application think about how you are going to structure your API's in advance. With open and freely available API's the old adage of "two heads are better than one" is more relevant than ever.
Who knows what one of your users/developers/IT freaks may come up with that makes your product better than you ever thought possible.