Everybody loves a good secret, and if you're reading this site then presumably you also love great rock songs. But were you aware of all the rock songs with secret guest singers or guitarists that are probably lurking in your record collection?

Together with our friends at diffuser.fm we've compiled a list of 20 tracks that feature anonymous or obscure -- but often easily recognizable -- contributions from rock stars ranging from Joe Walsh and George Harrison to Mick Jagger and even Michael Jackson.

You'll find out how these mystery collaborations came about, what motivated the artists to work together, and of course have the chance to hear the final project for yourself. Enjoy, and by all means please don't keep this list a secret -- tell all your friends!