Playr can now output XSPF playlists (which was nice and easy, by the way: there's a basic example playlist in the specifications) and this means it can also use the XSPF Web Music Player to play them in the browser.
There's a Flash/XSPF bookmarklet on the front page -- which you can use to directly play any page containing linked MP3s -- as well as 'Flash' links on the New Playlists page.