Thrutu is an Android app that helps users exchange information while talking on the phone. With one click, people can exchange their current locations, pictures either through the camera or from your gallery, and contacts. However, people might want to use this data-sharing capability to exchange other things during a call, too. Thrutu facilitates this through custom buttons via the Thrutu API.
ReadWriteWeb gave this one a pretty good write up. There, they talk about some of the buttons already written with the API:
PayPal - Send money over a phone call, instantly and securely. For example, when you call friends with a friendly reminder to pay you back, they can transfer the funds into your account instantly.
Meet Me - Quickly find the best place to meet in person. Meet Me suggests venues, like cafes or movie theaters, that are located halfway between the two callers. On Thrutu, you can instantly discuss your options and agree upon the destination.
Shared Search - Search and browse the web together. Either caller enters a word, and both can see and click on the results.
Doodle - Share sketches in real-time. Doodle is also integrated with sharing locations and photos in Thrutu, so users can add a personal touch, such as an arrow on a map or a heart or smiley face to a shared photo.
Coin Flip - Make decisions or settle arguments with the toss of a coin.
Mood Ring - A cute way to let the other caller know how you're feeling - without having to use words.
15 Together and Tic Tac Toe - Play classic games across the Thrutu platform, all while continuing conversations, encouraging competitive banter.
My Flickr and My Twitter - View pictures or read tweets together, with instant, shared access to Flickr and Twitter streams.
Wish You Were Here? - Share your local weather forecast. Whether on a tropical vacation or snowed in at home, callers can gloat (or complain) about their current weather while on the phone.
A lot of neat things have already been written with the API, but there is always room for more. There are actually two different API types: native Android applications, and a Web API. Both are pretty well-designed and useful.
The native buttons API has three different sorts of useful interfaces: Picker, for things that can be handled by default applications, JustSendIt, for things that don't launch an activity on the other phone, and Activity, for things that directly launch an Activity on the other phone. All of them require a Thrutu API key.