The theremin is one of the strangest music instruments ever devised -- it detects the movements and position of your hands in open space and produces sounds in return -- so it was just a matter of time before the iPhone received an amazing app that mimics its capabilities. Theremin I/O is that app, and after playing with it for longer than I'd like to admit (and scaring the wits out of my two cats in the process) I can safely say it does a fine job of replicating the ridiculous instrument's feedback.
Whereas an actual theremin utilizes metal antennas to detect the users hands, the app uses the iPhone's camera with the flash permanently on. A small section of the virtual theremin control panel on the iPhone screen shows how much light the camera is picking up from your hands, and you can get a feel for what parts of the screen produce certain frequencies.
It's a fun little app, and even though it doesn't make any attempt to follow the iOS 7 aesthetic (seriously, this app looks like Scott Forstall's fantasy), it's unique and that's what matters most these days.