Last.fm iOS and Mac OS X SDK is a development kit created Kevin Renskers. Using the SDK, developers can work with blocks, use only KissXML as a dependency, return values in correct data types, hook catching methods, and cancel operations. This kit is actively developed and maintained and used in the official Last.fm Scrobbler app. Other apps using Last.fm include MetroLyrics and Radio.com. Requirements to develop include iOS 4 and Xcode 4.4 or higher, an API Key obtained at last.fm/api, and Kiss XML.
ProgrammableWeb lists many popular Music APIs in the directory including Last.fm, 7digital and Spotify Metadata. This post features a few really creative, entertaining mashups that use a few popular Music APIs.
Sometimes it takes a little human intervention to make semantic applications easier to build. The Guardian newspaper has augmented its Open Platform API with unique identifiers for bands and books. In turn, the company has simplified the process of creating a mashup that uses multiple sources to focus on a single work, which may help its content spread farther.