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. 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.