First we have an iPhone guitar tuner web app. Which basically works more like a tuning fork than a tuner. But I guess in a pinch this could come in handy outdoors somewhere.

We came across the next video thanks to iphonenewsblog. There's apparently a web app for the iPhone that simulates a guitar. You get all six string and a few frets to play with. You use one hand to simulate strumming or plucking and the other hand to fret the notes. It even can simulate slides.