Dropbox API - and this could become quite an important toy for me in the near future - an API for Dropbox. Of course, most of the time I just need to send files back and forth, so simply using Dropbox is sufficient. But for some things, an API to access the metadata on Dropbox would indeed be interesting (one of my projects that has been on the back burner for a while would be an implementation of the Simpletext.ws service from Google App Engine on a normal Python service with Dropbox as the backend, for example).