Some of you may have heard of Apocalyptica, a bunch of guys that play Metallica and other metal songs on cellos (its pretty rad, check it out). Along a similar vein, I've stumbled across some other not so conventional tributes to Metallica:

Beatallica - Imagine Metallica doing Beatles covers. The singer sounds so much like James, its scary.
Harptallica - Metallica played by two harps
Pianotarium - Metallica played by an amazing pianist (heh heh....I said pianist)

enjoy...