I am not a professional guitarist. Like many of you, I can fake my way through a few chords and song melodies, but I couldn't play you any "tasty licks" or shred your face with my epic guitar wankery. I'm a hack, and I'm comfortable with that.
Humble skills aside, I do enjoy picking up the guitar at least a few times a week--just for fun and to keep up what few chops I have. My big problem is finding a way to play guitar without aggravating my wife, or waking my kid. I have an amp, but it doesn't have a headphone output. I have a slickUSB soundcard for my computer, but the setup is a bit involved for just messing around and it's not exactly portable.
When I heard about, a $39 headphone adapter and free app that works with the iPhone, iPod Touch, and iPad, I knew I had to check it out. The premise seemed simple: an adapter for plugging in your guitar and a pair of headphones; and an app that runs emulations of guitar amps and effects, which you can tweak to your heart's content. Best of all, the whole system is compact enough to fit in your pocket, provided you aren't using it with an iPad.