GuitarToolKit is one of those apps which most people wouldn’t use everyday, but when you’re about to play your stringed guitar-type instrument, its an absolute must. GuitarToolKit includes a tuner, a playable fretboard, metronome, and chord reference guide all of which can be use with support for guitars, basses, banjos, mandolins, and ukuleles.
With a clean, usable interface, and features galore, GuitarToolKit is an impressive offering. Although the price is a bit steep (for the full version), one could make the case that a non-iphone tool that does all of this so well would be much pricier. Find GuitarToolKit in the following locations:
Probably my most-used app on the iPhone, Instapaper (with its bookmarklet) lets you flag webpages to read later. It fully integrates with Google Reader, and the iPhone application caches your flagged articles for offline reading. One of my most favorite features is the text-only view, which optimizes the page you’re reading for the iPhone screen and eliminates all of the unnecessary distractions. 
Meter Maid allows you to tell it how much time you put into your parking meter, and it will remind you (with a Push notification!) when there’s only a little time left. For those of us who are surrounded by parking meters and hourly parking, Meter Maid is essential to avoiding those costly parking tickets. 
Color Stream is an app for designers, allowing users to create custom color palettes, either manually or by extracting colors from photos on the iPhone. While beautifully designed user interface makes using Color Stream quite fun, the export options make the app quite useful. You can export either to your photo album, email it, or find Flickr images that are made up of this color scheme.