Zillow has released an application for iPhone users that builds on their Zillow API, the Microsoft Virtual Earth API and the iPhone's GPS capabilities. The Zillow iPhone app detects your location and shows data about nearby homes, including market values, photos, and which ones are for sale. It's a great example of a mobile mashup application that uses multiple APIs in meaningful location-aware context.
Wheelmap.org has created a user driven resource for identifiying wheel chair accessible locations. The open source platform allows anyone to add accessibility descriptions and update old ones. The company provides the Wheelmap API for integration of this growing resource.