Johnson Bubble Level is a free application for Apple iphone and itouch version 3.1.3 or greater.  These smartphones/PDAs come with an accelerometer (the chip that allows your phone display to move when its tilted).  The application creates an image of a spirit level bubble vial on the screen of the phone.

• Works in three orientations:  flat phone (surface level), horizontal phone and vertical phone
• Calibration feature included
• Accuracy displayed to tenths
• Read "level" in various modes:  degrees, percent, inches/foot (fractional & decimal), mm/m
• Hold feature included
• Sound feature included
• Use it to straighten shelves, adjust hanging pictures, or cheat on your golf putts