There's a cd called Rock Swings out there, by a fella called Paul Anka (he wrote My Way for Frank Sinatra I think). Some of it is garbage but a lot of it is great, particularly It's My Life, Smells Like Teen Spirit, The Way You Make Me Feel and Jump. Black Hole Sun and Wonderwall are ok, but the rest are a bit ordinary.
Also, We're a Happy Family is a tribute to the Ramones, by Chilli Peppers, Rob Zombie, Eddie Vedder, Metallica, U2, Kiss, Manson, Green Day and a whole heap of others. I haven't listened toit in a while but its pretty good.
Erm, Nightwish doing Phantom of the Opera, Evanescence's Heart Shaped Box, I Will Survive and Perhaps, Perhaps, Perhaps by Cake, Blind Guardian's Mr Sandman, Spread Your Wings and Surfin' USA, Gunners doing Nice Boys, and a whole heap of others I can't remember now.
The absolute worst cover I've ever heard would be Rolf Harris singing I Touch Myself, on Andrew Denton's radio show a few years back.