Some of these covers can be found on the 4 disc set "Unearthed" which comes with a 5 star rateing from HndsmCelt

One particular jewl on it is a duet with Johnny and Joe Strummer doing Bob Marley's Redemption Song. The rest turn up on the 4 or 5 recording entitled CASH (with various subtttles). Having some where arround 25 or so fo Johnny's albums I'd have to sa the ONE collection I could not live with out is CASH Unearthed.
I'm late to the party since I rarely come to this area, but whatever. This is a wonderful collection. And I'm so with you about Mercy Seat, although I've never heard Cave's original and Redemption Song. I'm also a huge Clash fan and named my dog after Joe Strummer and the first time I heard it when he comes on for the second verse it was like hearing from a long lost friend, even though I knew they did this together I was momentarily stunned. What a rough stretch of time between 2002 and 2003, we lost so many musical greats.
As far as other covers go, here's a partial list of favorites
Joan Jett - Do You Wanna Touch
The Ramones - Needles and Pins
Sid Vicious - My Way
7 Seconds - 99 Red Balloons
The Sonics - Louie, Louie (actually numerous other versions are better than the one credited as the original)
The Clash - I Fought the Law, Brand New Cadillac (still a fan of both originals)
Stray Cats, Throw Rag - Please Don't Touch
The Commitments - So they aren't a real band and it may be sacrilege to say but I prefer their versions of Try a Little Tenderness and Dark End of the Street among others.
Just about anything Me First and the Gimme Gimmes do (poppy punk cover group).
That's just getting started though, I am actually a big fan of cover songs.