Programmable web is a great resource for tracking APIs and mashups and has become the defacto registry for web APIs. The site provides a wealth of statistics tracking back to 2005 when the site was started by John Musser.
You can see that just 5 years ago there was only 100 odd APIs out there, before that eBay, Amazon and SalesForce were the trailblazers. Whats interesting is the acceleration and the types of APIs being released. With the exception of Amazon APIs started out exposing application data, then social media, then digital content, then well everything else, It’s pretty amazing that by the end of this year there will probably be over 5,000 APIs giving you access to everything from making conference calls, to storing files in the cloud, to calculating your carbon footprint with AMEE.
API as a Product
The interesting thing is that for many new companies their API is the product, Amazon Web Services is the grand daddy here, but Twilio is one of the coolest making telephony services available to everyone. Face.com is another which offers facial recognition APIs, and pretty much all of the infrastructure services like Loggly, MongoHQ and SendGrid are all selling developer APIs to do stuff you used need infrastructure for.