In Flames

Yeah, the In Flames' cover is really good. Very catchy chorus.

Another cover I find really really good is Twisted Sister's "Burn In Hell" performed by Dimmu Borgir on 'Puritanical Euphoric Misanthropia'.
 
I enjoyed the Dimmu cover of "Burn in Hell" as well. The funny thing is that the song always reminds me of Pee Wee's Big Aventure. My favorite cover that Dimmu did was of Accept's "Metal Heart."
 
My favourite cover is by Pyogenesis (A lousy band indeed, but the cover is great.) and it's Toto's "Africa".
They made it jungle, with some really heavy guitars. It's hilarious.

And Diablo cover of Abba's "Dancing Queen" kicks ass. I just don't know if it's on the record, but it was on their demo. For you who don't know, Diablo sounds a bit like In Flames.

Also great are Nirvana's "Smells like teen spirit", covered by Tori Amos (!) :) and of course "Mr. Sandman" covered by Blind Guardian!
 
Of course one interesting cover is by Covenant (nowadays known as The Kovenant), Babylon Zoo's "Spaceman". The original song sucks, and the cover isn't much better, but still it's an interesting choice. They could've made it much better, though (I could've.. :D )
 
One of the best covers is "Marijuana" by Brujeria.
Actually, it's a parody of "Macarena" that lame over-played song from a few years ago.