Tutor for Compass for the iPhone
This is one of the shortest tutorials on the site, but one that highlights a couple features that are easily forgotten or not even known about with the iPhone. You can use the iPhone as a compass and as a level. The compass shows North, your longitude and latitude, and altitude. With the level you can see if a table is level by holding the iPhone horizontal, but when you move it vertical, it will help you level a picture on a wall. A couple of powerful tools that are easily forgotten about.