I find programmableweb.com to be a Really Useful resource for looking for APIs that are out there on the web.

There's a great dashboard showing various stats (most popular, API 'type' breakdown) available here. From their latest blog post discussing this stuff:

Just a quick note on our API directory: we now have 550 APIs listed. And although the Google Maps API is still used for more of these than any other, less than a quarter of the mashup entries in the last 2 weeks are about mapping. As a matter of fact, the distribution across topics lately has been notably diverse with the top three non-map types being video mashups, social mashups and photo mashups.

The mashup matrix is an interesting insight into 'what happens if you combine x and y', though the visualiser itself needs a bit of work (maybe some Silverlight...)