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.
Social Money, an online goal savings platform provider, has announced the launch of the private beta version of the CorePro API which allows companies to integrate savings accounts into online and mobile applications. The CorePro Platform is the first product that Social Money has developed that offers an open API and was built using elements of the company's SmartyPig platform.
When it comes to map mashups there's no doubt that Google Maps leads the way. On ProgrammableWeb we list 1,964 Google Maps mashups, almost four times as many as any other API. This popularity makes utilities built on top of Google Maps especially interesting, such as this contextual menu control, which pops up a menu when the user right clicks.