RdioToday at the South by Southwest Interactive festival in Austin, Texas, Rdio, the unlimited, on-demand social music service, introduced its API to the public.

The API gives developers the ability to create web applications that can search, access and play all of the artists, songs, albums, playlists, and top charts in Rdio’s catalog.

Developers who are looking for a way to incorporate music into their applications can now do so through Rdio’s API, and will be able to monetize their applications through Rdio’s affiliate program which pays commissions for referring new subscribers. Competing music subscription service Rhapsody also offers an API, but we’ve yet to see many interesting uses of it, perhaps because Rhapsody doesn’t have a similar affiliate program.

Rdio, which was founded by a co-founder of Skype, is a music service that lets subscribers listen to as many songs as they want and share new music with friends. The service offers a $4.99 monthly subscription for unlimited listening through your web browser, and a $9.99 plan that includes mobile music streaming and synchronizing. Rdio subscribers build and share their online music collections from a catalog of over 8 million songs and can look at the listening activity and playlists of other users.

Todd Berman, VP of Engineering at Rdio, says, “Ever since our launch six months ago our API has been our most requested feature. Developers who have been looking for a way to integrate music into their web applications now have a way to do it easily, legally and accessibly.”

Rdio’s API is currently being used by AOL (for its newly launched “Play by AOL” application), Grab.com and Tweetlouder.

The company, which was founded by Skype co-founder Janus Friis, is headquartered in San Francisco. Rdio is funded by Atomico and Janus Friis, through his investment entities Mangrove Capital Partners and Skype.