I visited a Star-Wars exposition this summer and I noticed that Yoda was wearing something strange around his neck:
This looks like a small book, what is it?
It's a blissl, a musical instrument similar to a pan flute.
This information was revelled the February 28, 2002 on starwars.com's Ask the Lucasfilm Jedi Council. The original page is unavailable, but it's archived here :
Q : What is that thing around Yoda's neck?
A : Yoda wears a blissl around his neck, which is similar to a pan flute.