Last.fm besides being one of the most popular online radio and music communities, also has one of the most popular online music APIs. Possibly the most popular. Each month we continue to see a variety of new music-themed mashups added to our directory built on the Last.fm API. In the first three weeks of this month alone there have been 5 more Last.fm mashups added. That means that out of 231 music mashups at least 84 use the Last.fm API. And not that developers don't have other options given that there are 44 music APIs to choose from.
As the amount of processing power that can be affordably invoked via the cloud continues to increase, applications that were once thought too impractical to build are suddenly quite feasible. A good example of that is AppStream, a new service that Amazon is beta testing that allows graphically and resource-intensive applications to be extended out to any number of devices that previously would not have had the processing capabilities to run them.
The Stipple API is free, has no request limits as of yet, and OAUTH 2 will be supported in the future. According to its website on the API, it is "inspired by REST". Stipple announces its rare downtime, which is usually used to migrate data, via its twitter account.