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.
American Express found another way to convert social media to cardholder transactions at the SXSW conference using the still-private Smart Offer API. Amex's “Sync. Tweet. Save.” social marketing promotion positions the credit card for the social media generation beyond membership points.