Someone elsewhere told me only today that William Shatner did a version of Tambourine Man http://uk.youtube.com/watch?v=_0hTtsqiFCc

Harry Nilsson's version of "Without You" was very famous, but I have a soft spot for the original from Badfinger, written by Pete Ham and Tom Evans - partly because of what came after. "I can't live, if living is without you/I can't give, I can't give any more" and both of them separately committed suicide in 1975 and 1983

Johnny Cash did an effective cover of U2's One.