3. Bubble Level
This $0.99 app
from CirkelSoft leverages the iPhone’s accelerometers to turn it into a fairly effective bubble level. The downside: you need to calibrate the app first with a real level to give it any accuracy. But after calibration it’s always there with you, even when you show up at a friend’s house to hang shelves only to discover that he doesn’t own a level. Doh!