This is gonna be a long, vastly incomplete list. But here goes:
Turisas - Rasputin (this is a must)
Anything Van Halen and Jimi Hendrix covered were infinitely better than their original counterparts.
Judas Priest - Green Manalishi, Diamonds and Rust
Skid Row - Strength (The Alarm), Little Wing
Dream Evil - My Number One, Let the Killing Begin (Arch Enemy)
Steel Panther - I Want It That Way (Backstreet Boys)
Metallica - Almost anything on Garage Inc.
Firewind - Breaking the Law, Believe in Nothing (Nevermore)
Final Stage - Gods of War
Queensryche - Gonna Get Close to You, Scarborough Fair, Syncronity 2 (really like their take on this)
Pink Cream 69 - We Will Rock You, Truth Hits Everybody
Powerglove - Red Wings Over Baron (Final Fantasy 4)
Primal Fear - Metal Gods
Fozzy - Their first 2 albums
Steel Dragon - We All Die Young
Symphonity - Anyplace Anywhere Anytime
Theocracy - Any of their Christmas songs
Gotthard - Hush
Great White - Once Bitten, Twice Shy
GNR - Knockin' on Heaven's Door
Helloween - Eternal Dark
Heaven's Gate - Always Look on the Bright Side of Life, Animal
Fates Warning - Sign of the Southern Cross
Also, does Beatallica count?