The Instamojo Ruby SDK by Instamojo allows developers to integrate the Instamojo API's methods for creating, editing, and deleting offers into their Ruby applications. This SDK also supports listing, updating, and getting details for payments, payment requests, and refunds.
MyBantu is an interesting little mashup, available as an iPhone app, an Android app, and a website. Its goal is to make shopping for anything, especially restaurants and movies, easier and more social. It uses Yelp API, Netflix API and various others to make it happen. It's very similar in idea to Siri, the "ultimate mashup" that Apple later gobbled up in the largest mashup acquisition ever.