I had an idea a while ago to make a "rain chime", similar in concept to wind chimes. It would make noise when struck by rain in a semi-musical way. Then I thought that can't be a new idea, so I went to search for rain chimes online. What I found appear to be devices that sound like wind chimes and don't require wind or rain. I wonder why they call them rain chimes, then.
Mokalus of Borg
PS - It's not a new idea even the way I imagined it.
PPS - See this post at Halfbakery, for instance.