There are two handwritten notebooks of mostly keyboard music known as the Anna Magdalena Bach Notebooks. Anna Magdelena was the second wife of Johann Sebastian Bach and also a musician. The first notebook consisted of pieces composed and handwritten by Johann Sebastian which he presented as a gift to his wife. The second notebook was mostly handwritten by Anna Magdalena herself (with occasional help from others), and the pieces were composed by various members of the Bach family, their students or musical friends. For many years the pieces in the second notebook were all assumed to have been composed by Johann Sebastian, and even in 1950 Wolfgang Schmieder catalogued the pieces as being Bach's and gave them "BWV" numbers. More recent analysis by scholars has discovered that J. S. Bach only composed a few of the works, and they have now attributed some of them to the relevant composers. The composer of this Musette in D (no. 22 from the second notebook) is still not known, though the sheet music, midi and mp3 files can be downloaded using the links on the left-hand menu.
Also on the left are links to other pieces from the A. M. Bach notebooks. Further examples of these works, which make a great introduction to Baroque music, are available from Sheet Music Plus in the US or The Music Room in the UK.
Here is a video of this music, or scroll down the page to see the sheet music.