The Klout Objective-C SDK by Camille Kander connects with the API to integrate analytics features that allow to measure business reputation. Based on sentiment analysis, developers can help users to have a voice when creating content for a specific website. HTTP and JSON formats are available for API interaction.
Check-in app Gowalla puts a fun twist on location sharing by helping its users discover wonderfully-designed virtual items that can be found, swapped, and dropped all over the place. Developer Ben Dodson used the Gowalla API to put together Wallabee Travel Edition.
Looking for something cool to do but have no idea where to start? A tool like LairOut could be helpful in this situation. It's basically a web-based event directory that allows users to access information on a number of events happening in and around their location. LairOut also provides the LairOut API that allows developers to access this functionality.