It's always exciting to see those outside the web industry using an API to improve their sites. NY Times First Look blog has a great example of what Dallas Public Library has done with the Best Sellers API.
Diffbot is one of the coolest new ideas on the internet. The service brings monitoring to the web in a new and interesting way. The company is just about to release a whole slew of projects built on its collection of Diffbot APIs for following changes to web pages and RSS, as well as extracting clean text from websites.