I whipped this app up to start experimenting with pitch detection, and also to test live audio input. It performs a naive (zero-crossing based) pitch detection algorithm in realtime. It works best today with whistling (which has a clear, simple waveform); it needs some severe help with more complex waveforms, even like guitar. I'll get back to this eventually. :) There are much better tuners out there right now - like Craig Spence's work.
Check it out, feel free to fork, submit pull requests, etc.
-Chris