AirVox is somewhat like a Thermin, Alesis AirSynth or Roland D-beam for iOS. There are 49 presets to play with that take you through the different scales and available hand motions. $2.99 in the App Store (link).
“AirVox is a unique and precise musical instrument controlled without touch — only using hand gestures and motions. Interact with AirVox via the front facing camera. Our real-time computer vision technology “sees” your hands’ movements and turns it into musical notes and tones. YOU are the instrument! No previous musical experience is required to control Airvox. Its ease-of-use and many built-in features make music creation instant, and more importantly, fun! But AirVox is also a precision instrument. You can perform with the expression and phrasing usually heard in traditional instruments.” – yonac.com
If you can’t afford a pretty Moog Etherwave Plus ($519) and you still want to sound like Portishead (I know it wasn’t a real Theramin) maybe Thereminator (iTunes link) for the iPhone will do the trick for you. It’s $3 and has over 200 five star ratings.
“The theremin, originally known as the aetherphone / etherophone or termenvox / thereminvox is an early electronic musical instrument controlled without contact from the player. It is named after its Russian inventor, Professor Léon Theremin, who patented the device in 1928. The controlling section usually consists of two metal antennas which sense the position of the player’s hands and control oscillators for frequency with one hand, and amplitude (volume) with the other. The electric signals from the theremin are amplified and sent to a loudspeaker.” – Wikipedia.org