I'll need to give all these links a proper watch later on. Cash did some great covers, seem to remember his version of Soundgarden's 'Rusty Cage' being really good too.
Never been a fan of Black Sabbath (I know, it's not cool to say that, I've just never really been keen on Ozzy's voice) but I have always loved War Pigs mainly thanks to Faith No More's cover being on a VHS tape I had as a kid - http://www.youtube.com/watch?v=DhIMf61NeB8
In fact, they've been responsible for a lot of cracking covers over the years. I was at one of their gigs a couple years back where twice during the show they broke off mid-song to play a lovely rendition of the Eastenders theme (found a video of it here - https://www.youtube.com/watch?v=2xAU-9flSCQ#t=148
The first time was funny but the fact they did it twice during pretty heavy songs was just genius, totally broke me.