Drop D Tuning. Minus the little intro (which may or may not be played on a keyboard, but 
could easily be picked up on guitar), this is really all the guitar does during the song.

E|---------------------------------|
B|---------------------------------|
G|-----------------------------5-5-|
D|-1-1--1--1-1--1--5-5-5-5-3-3-5-5-|
A|-1-1--1--1-1--1--5-5-5-5-3-3-3-3-|
E|-1-1--1--1-1--1--5-5-5-5-3-3-----|