Bach's Italian Concerto, second movement. Click here for first movement.
I love this piece. I first learned it when I studied with Naomi Sparrow as an undergrad, and I was surprised to find that it was still sort of "in my fingers" twenty-some years later. However, I think I had to un-learn a bit--I've come to the conclusion that when I was young, I was musically an idiot with good hands. I'm musically smarter now, but my hands are not so good.