Magnetic Stud Finder If you're hanging anything heavier than a walnut, use your phone's magnetic capabilities to make sure you drill or hammer into a sturdy stud.

iHandy Level Free Then level it off. This simple app uses your phone's accelerometer to help you make things plumb and perfect.

TapeRuler This app works like a measuring tape and is most accurate over short distances. I wouldn't use it to measure a whole wall, but to get small things right, it's easy.

Handy Construction Calculators Planning something a little bigger than a shelf? Use this set of calculators to figure out specifics like area and estimated cost for a whole slew of building materials. It costs a bit, but for the chronic DIY-er, you can call it an investment.

Construction Master Pro This one tops out the price range at $20 and is really for the most serious home improvement nuts of them all. It's gotten great reviews by construction professionals on a lot of websites. This Android version includes even more features than the iPhone version, including professional calculators and detailed images with definitions. Look at my link next to this article for a detailed review.