iOS: Chances are you don't have a ruler on you at all times, but you probably carry your phone with you everywhere you go. Acoustic Ruler Pro is an app that converts your iOS device and its headphones into a ruler for quick measurements.By emitting a sound and picking it up with the microphone, the app can give you a pretty accurate measurement on the go. It comes with two uses, dual device and single, each with different measuring distances. If you have two devices, you can measure up to 25m and with one, you can get up to a little over 1m. If you're using one device, there's a calibration walkthrough using headphones with a mic, as well as one for without. Using headphones without a built-in microphone will extend your distance, as there's usually more cable between the headphone and microphone on the phone.
Dual-device mode works by putting one device in master mode and the other in slave. You'll calibrate by putting them next to each other and it gauges the distance between the two devices from there. Due to its sound-reliant nature, this works best in quiet environments and while you probably don't want to be making any important cuts using this, it does provide a quick measurement for on-the-go estimations. Acoustic Ruler Pro is a $0.99 download from the iTunes App Store.