For a long time, I didn't know Johnny Cash's Hurt was actually not his but a Nine Inch Nails song.
And while I think that the NIN version is a pretty perfect conclusion to an excellent album (The Downward Spiral), there is something about Cash's version that makes it unique - a frailty and the feeling that the guy had an accute understanding of his own mortality (he died within a year of releasing his version of Hurt) that give it a beauty the NIN version just can't have. Trent Reznor actually loved this version too, after initial reservations that it would be "gimmicky":
I pop the video in, and wow... Tears welling, silence, goose-bumps... Wow. [I felt like] I just lost my girlfriend, because that song isn't mine anymore... It really made me think about how powerful music is as a medium and art form. I wrote some words and music in my bedroom as a way of staying sane, about a bleak and desperate place I was in, totally isolated and alone. [Somehow] that winds up reinterpreted by a music legend from a radically different era/genre and still retains sincerity and meaning – different, but every bit as pure.