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.
Roistr is a project that’s smaller than our typical API feature. It shows the falling barrier to entry in the API world even in a the demanding academic area of natural language processing and semantic analysis. The primary Roistr API function is a “best match” engine. It offers an interesting use case to demonstrate the relevance of the service: take a Twitter username and use tweets from this account to find content most relevant to the user’s interests.