Great for singers, and fun too

The Pitch Pipe app has a great interface. Plus, it has a feature on the reverse side (when you press the "i" at the lower right) which allows you to have it immediately start playing the previous note. Its optional, but if you want it to automatically play the same note as the last time you used the app, it saves you some time.

It works on the iPod touch through the headphone jack, so you can hear it in your earbuds. No microphone is required.

Audlee about Pitch Pipe