If you own a guitar and an iPhone, you should also own this app. Its main function is as a tuner (it won't work with an iPod touch as it lacks the iPhone's microphone), but it also features a metronome and a handy chord lookup function.
The tuning feature works very well. By default it does standard EADGBE tuning, though a quick tap of the Mode button switches you to drop D tuning if you'd rather. If that sounds a bit restrictive, there's a bewildering list of alternative tunings just a tap away. The list is extensive; now you have no excuse for your guitar being out of tune.