I'm actually a big fan of cover songs and have a small collection. My favorites are:

Impelliteri - Since You've Been Gone (Rainbow)
Girlschool - Fox on the Run (Sweet)
Electric Boys - Black Betty (Ram Jam)
Kik Tracee - Mrs. Robinson (Simon & Garfunkel)
Angra - Wuthering Heights (Kate Smith)
Beau Nasty - Love Potion #9 (The Clovers)
Axxis - Na Na, Hey, Hey, Goodbye (Steam)
Anthrax - Got the Time (Joe Jackson)
Hanoi Rocks - Around the Bend (CCR)
Poison - Cover of the Rolling Stone (Dr Hook)
Thyrfing - Over the Hills (Gary Moore)
Sanctuary - White Rabbit (Jefferson Airplane)