A440 Tuner is a simple app that gives a musician a reference Concert A (440MHz) pitch in any combination of 3 different forms: as played by a real oboe, as played by a synthesizer modeled oboe, or as a sine wave.

This app is not a meter or strobe or chromatic tuner – it simply produces the Concert A reference pitch

http://appshopper.com/music/a440-tuner