The Python Flickr API kit is a feature-rich Python interface to the Flickr API. It comes with a caching framework and remembers the user's authentication credentials automatically. It supports a variety of communication protocols, and offers different XML parsers to suit different platforms and preferences.
Mashup contests can be a good way to reveal the variety of applications that are possible when the developer community is provided with a quality API. At the recent BarCamp London 5, Stefan Magdalinski, CTO of MOO.com, ran a weekend-long MOO API hack competition. The MOO service and API let users submit custom designs that can be printed onto products like business cards, sticker books, postcards, etc. These products can then be sold through an online store.
Photo editing service Aviary has added a new HTTP-based API that allows developers to pass photos through filters, effects and even basic editing operations. Aviary "soft-launched" the new Aviary Effects API last week at SXSW, when we interviewed Aviary's Michael Galpert (video below). The new service could be used to create photo-sharing services like Instagram, CMS plugins to aid workflow or within social applications to create a standard look to all avatars.