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.
The Independent Online Distribution Alliance (IODA) has just announced its IODA Open API, currently in beta. The API offers many of the typical resources that you would expect from a music metadata service, with a few extra goodies like these album attributes: UPC code and primary label. Comparing album API results between IODA Open and iTunes, IODA is packing a lot more information into the response.
The site claims a "huge collection of hit lyrics." Your next music application can tap into the database with the new ChartLyrics Lyric API. You can use it to search in several ways and, more importantly, obtain the text that makes up the words to popular songs.