From their site: The Google Gadgets API provides a way to put third-party content onto the Google homepage. The main use of the Google Gadgets API is to create gadgets (mini-applications) that users can add to their Google personalized homepages and other Google properties. For example, you could create a photo album gadget, or a gadget that displays an interactive game. The API is designed to be simple enough so that anyone can turn his or her web page or application into a gadget.
Our API directory now includes 54 widgets APIs. The newest is the Backplane Server API. The most popular, in terms of mashups, is the Google Homepage API. We list 101 Google Homepage mashups. Below you'll find some more stats from the directory, including the entire list of widgets APIs.