Ooh - the Continuum Center for Health and Healing has started to use leeches in a treatment for osteoarthritis in the knee. Leeches have been used in other kinds of surgery, but apparently this is the first time leeches have been used to treat the knee. No one is sure why leeches are able to give osteoathritis sufferers relief, so the clinic is looking for more volunteers (it's run by Beth Israel, so it's not some quack scheme) to take part in the study - the volunteers do need to shell out $600, though.
Last year, there was a great New Yorker article about the leech industry; sadly, it's not online, but this is the British company featured in it, Biopharm. And this illustration is of Larry the Leech from Discovery Kids, where there is an explanation of what leeches do (also, here's the Wikipedia entry on leeches). And remember that part in Stand By Me with the leeches?