Android: If you're curious about how tall or how far away a building is but don't have the means to measure it, $US0.99 app Advanced Ruler Pro will use your phone's camera and accelerometer to give you an estimate.
When you first download the app, you'll want to measure how high your eye level is (with a measuring tape), and then always hold the app at eye level when you use it, ensuring you get the best estimate possible. After recording this height into the app (by tapping the "camera height" label in the bottom left of the screen), you can measure, for example, the height of any object by pointing the crosshair at the top of it and tapping the screen. Make sure, of course, that you just angle your phone's camera and not actually lift it up as your instinct would tell you to do. Once you've tapped the screen, the app will freeze the "object height" label so you can record it.
The app works with both imperial and metric scales, and uses similar methods to measure distances and lengths as well as heights, so no matter what you're trying to measure, Advanced Ruler Pro has you covered. Keep in mind it's just an estimate, so you wouldn't want to use this for anything precise. If you need to know, say, how many 1.8m lengths of rain gutter you'll need, though, this will give you a pretty good idea with minimal effort.
Advanced Ruler Pro is a $US0.99 download available for Android devices in the Android Market.