I love the Cash version of Hurt, almost as much as the Kermit version.

Some of my favorites, not out-of-genre though...
1000 Homo DJs (Jourgensen and Reznor, et al) doing Black Sabbath's Supernaut
https://www.youtube.com/watch?v=Xjho-zVgDXo

Tool doing Led Zep's No Quarter kicks an vast quantity of butt:
https://www.youtube.com/watch?v=fA7sHz2m_Cs

Sepultura doing Motorhead's Orgasmatron - can't find a decent vid unfortunately.

Out-of-genre, I guess this is a good one, Dynamite Hack's take on Easy-E's Boyz In the Hood (some language)
Spoiler Alert, click show to read: 


Anthrax's version of the Venture's Pipeline is rolling...
https://www.youtube.com/watch?v=l18cy8S8WZU

The String Quartet Tribute groups have done a ton of stuff, Slayer's Raining Blood may be even creepier with the acoustics
https://www.youtube.com/watch?v=HGVNAdgLTCg