Niklas Zennstrom and Janus Friis, the founders of Kazaa (as well as Skype and Joost), have unveiled a new (entirely legal) music streaming service - an increasingly crowded market these days. Called Rdio, the startup will charge $5 to $10 per month for unlimited access across PCs and mobile devices. This same premise has long been available in services offered by Microsoft, Rhapsody, Napster and countless others.

Rdio is run out of San Francisco by 22 employees, and it'll open this week in an invitation-only preview with widespread availability expect later this year. Despite the rocky history between Kazaa and record labels, at least one has agreed to work with Zennstrom and Friis. "We resolved the past," said the Warner Music Group. "These guys are focused on the future."

On the surface, Rdio looks like any one of the other all-you-can-eat music services, but it promises to break from the norm by adding social features, such as letting users see what their friends are listening to. People can also view a list of the most popular music among their contacts. There's also talk of linking up with Skype somehow, but we're not sure how that would play out.