I whipped this app up to start experimenting with pitch detection, and also to test live audio input. It used to perform a naive (zero-crossing based) pitch detection algorithm; now it uses a naively-implemented auto-correlation algorithm in realtime, so it should work well with most monophonic waveforms (although strong harmonics will throw it off a bit). It works well with whistling (which has a clear, simple waveform); it also works pretty well to tune my guitar.
Live instance hosted on https://webaudiodemos.appspot.com/pitchdetect/.
Check it out, feel free to fork, submit pull requests, etc. MIT-Licensed - party on.
-Chris