If you put this on a Boehm flute, it turns it into a chromatic Low D whistle. Pretty darn cool.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=2576302909&category=10183
(cross-posted on the whistle board, as I think there are people who read one board or the other exclusively, and I think this item is really cool - I have two.)