From the Yahoo Music Blog:
[Webjay is] a site for easy creation and playback of playlists from the Web. Users can create playlists using music/audio/video from around the Web (with a simple Web form, from scraping a Web page, or with a fancy Ajax interface created by a 3rd party using Webjay APIs), share them with others, include them on their Web sites, browse other users playlists, play the playlists in any media player, or cannibalize the playlists to create new ones.
The blog post adds that the Yahoo Music Engine will incorporate some of these playlisting features and open API's in the future.
Btw, a "social" music recommendation service that I literally downloaded today and have yet to use is Last.FM.
You get your own online music profile that you can fill up with the music you like. This information is used to create a personal radio station and to find users who are similar to you. Last.fm can even play you new artists and songs you might like.
Of course, my love and enjoyment for Pandora continues.