Most of my favorites have already been posted - so here is something a little different...
Apocalyptica is a Finnish cello quartet, that has created some cool arrangements of Metallica songs:
On a classical note, Metallica did some incredible performances with the San Francisco Symphony:
And finally, a live mash-up of Enter Sandman and Run DMC's "Tricky", performed by Smash Up Derby at the recent YNOT Summit adult conference Opening Night party:
ADG