Our API directory now includes 54 calendar APIs. The newest is the InfoPark Web Calendar API. The most popular, in terms of mashups, is the Eventful API. We list 47 Eventful mashups. Below you'll find some more stats from the directory, including the entire list of calendar APIs.
Google has made several announcements about APIs recently, including the launch of the Content Experiments API, the launch of the new Cloud SQL API, and the availability of the CalDAV and CardDAV APIs to the public.