Wikia is a collaborative publishing platform that enables people around the world to discover, create, and share content on any topic. Wikia also provides the LyricWiki API, a service which provides users with lyrics to many popular songs from a variety of artists.
This free API can be used via SOAP or REST, though the SOAP functionality is currently more robust. Lyrics can be returned as XML, JSON, HTML, or plain text. Authentication is optional when using the API via SOAP (and not available when using REST), and can be used if you want to associate any webservice changes that you make with your user-account.
Please note that due to licensing restrictions, the API can only directly return a small portion of the lyrics directly - which is useful for determining if you have a correct match. All responses also contain a URL to a corresponding page with the full lyrics.