The Localytics Xamarin SDK by Localytics allows developers to integrate the Localytics API into their applications built with Xamarin, which allows developers to write native mobile applications in C#.
For a couple years I've thought about creating a geo-located tagging app. The idea is that anyone could use it like Delicious, but for tagging anything they want in the real world. Learning a mobile platform and creating the backend was always too much trouble for what is really just a fun little project. Now, I could write it in potentially less time than it's taking me to write this post. The Human.io API lets developers create mini-apps within its iOS and Android apps, with access to the camera and user location.
Shoutlet, enterprise social marketing platform, has added a contact form API to its plethora of social media services. The API allows developers to gather contact information (e.g. data from an online sign up form) and automatically update the Shoutlet contact database with the pertinent information.