Don't laugh. This young man gives a remarkable rendition of the third ("Presto") movement of the "Summer" concerto from The Four Seasons.
If you enjoy this, be sure to also view the accordion versions of Rimsky-Korsakov's Flight of the Bumblebee and Bach's Toccata and Fugue in D Minor.
YouTube link.
Update 10/14/09 by Alex - the guy is Alexandr Hrustevich, and you can see many more excellent clips on his YouTube profile page.
wow.