I had a serious vibration problem when I put some 17cm Kenwood speakers in the rear of my Mk3. I sprayed Dynamat all over the door panels which seemed to cure the worst of it whilst also giving the door panels a pretty neat foamy effect. I was expecting it to look horrendous, but it actually looks pretty good.
More interestingly, I recently swapped my Clarion head unit for a Kenwood KDC-7024M, which eliminated vibrations on all but the most troublesome bassline (I'm thinking the intro to Seven Nation Army here).