While more attention lately has been on mashups and other applications folks are building on top of web service APIs, the number of APIs themselves has steadily been growing. As of yesterday the count here just passed the 150 mark. They run the gamut from commercial-grade, well-documented APIs to minimalist, barely supported ones.
If you want to keep track of what's new use the API Feed to follow added or updated APIs. You can also click on the headings of the API list to sort by date or category. Sorting by category gives a bit of food for thought with regards to David Berlind's point that differing classes of APIs are going to provide the basis for the Internet as the new OS.