We here at Beaconfire have used the Flickr online photo sharing website as a cost effective (ok, free) way to manage the back end of photo-based applications for clients. Flickr has allowed us do things that previously would have required a completely custom build and cost considerably more money. In these projects, we use the Flickr API to allow us to interact with photos stored in Flickr.
Flickr just launched a new site that collects all of the information needed for this kind of stuff together in one place. code.flickr includes a Developer Blog, ticket tracker and public SVN repository (for browsing the source code), and a forum with groups dedicated to the API and to the uploader.
Most of this information has been around for some time, but it’s nice to have it all in one place.