Just heard Dar Williams' cover of Pink Floyd's "Comfortably Numb" on the radio, and I think it actually might be one of those covers that surpasses the original song. I know that sounds like sacrilege, and I might be saying it only because we've all heard the original about 1000 million (i.e., a billion) times.
Williams' version is haunting, beautiful, lonely and soaring, all at the same time. Judging by the very little I've heard of her stuff, she's a pretty versatile singer; here, she has the voice of a sad angel.
Comfortably Numb (Windows Media sample)