Longhorn by BullPay provides real-time wallet balances, all historical blockchain transactions, and more on 5 blockchains and 1 testnet, bridging the communication gap between blockchain node networks and blockchain-related applications with a unified API.
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.
Podio is a web service that aims to provide a customizable online workplace. There are a lot of apps, and you can set up and position your dashboard as you please. However, sometimes the app you want isn't in the set. In that case, they also offer the Podio API to code your own custom apps for use within your Podio workspace.