I've heard of the one in Zennor - apparently, if you climb on it without rocking it 9 times at midnight, you become a witch! If you're a woman. Or something. I think it's mentioned in Robert Hunt's "Popular Romances of the West of England"
Also there's one at <a href="http://www.themodernantiquarian.com/site/1718">Helman Tor</a> - which, incidentally, I didn't find!
There do seem to be a lot in Cornwall!
G x