Folks, I'm sorry I haven't been posting much of late. I can only appeal to the demands of academia again. In the meantime, here is another favorite piece of music: the opening sinfonia to "Wir danken dir, Gott, wir danken dir" (we thank you, God, we thank you), by Johann Sebastian Bach, which is a reworking of a violin partita. I haven't been able to find a version of this on YouTube that I like, since they all place an emphasis on the organ, and my few years of attending church as a kid ruined me for organ music. Fortunately I found a version on Grooveshark that is all strings -- in fact, I think it's the same version I originally heard and loved which may explain why the organ versions don't move me as much.
This piece epitomizes baroque music for me. It's just magical. If I had to point to one piece of music that captures everything I love about classical music, this would be it. Take a listen.
Update (23 Oct): For comparison, here's the violin partita he culled it from (#3, Preludio):
Like my wife says, it's hard to believe all that is coming from one instrument.