Lead Singer James Hetfield invited Lemmy Kilmister from Motörhead on stage for a couple of songs.
Lemmy and the boys performed two Motörhead tunes: 'Too Late Too Late' and 'Damage Case'. Metallica covered four Motörhead songs on their Garage Inc. CD/Album.
13 - Overkill
14 - Damage Case
15 - Stone Dead Forever
16 - Too Late Too Late
I've found a YouTube video of the performance. The best part is when James is chatting about Motörhead (calling Lars, the drummer, the biggest Motörhead fan ever) and the person with the camera finally realizes, James is about to introduce Lemmy and invite him on stage.
The guy freaks out... and the rest of the audience goes nuts! Check it out.
The audio for James' introduction is fine, but when they start playing it gets a little muddy. However, you can hear Lemmy fairly clearly when he belts out 'Got no time for / Move over for a Damaged Case'.
Here's another write-up on the concert and appearance by Kilmister by contactmusic.com.