I'm not sure where I remember this riff from, but I just sort of found myself
playing it one day (It might be from the first Baldur's Gate), so here it is:

e---------------------------------------------------------------------
B----------------------------------------------------------0-1-0------
G----2-2-2-----2-2-2-----0-0-0------0-0-----2-2-2------2-2-------2-0-2
D-0---------0---------------------------------------------------------
A----------------------3---------3--------0---------0-----------------
E---------------------------------------------------------------------
     --t--     --t--     --t--              --t--

repeat several times. 

It's good to warm up with this, because then you can solo off the pentatonic
minor scale and really be ready to play.  It also sounds good with a grunge
pedal or on a higher octave, or in any key.


Have fun, peace, and MetallicA rulz!