iOS: Sometimes you need to get a quick measurement of something and you probably don't carry a ruler around, but of course, there's an app for that. VisualRuler gives you a decent estimate of an object's dimensions using your phone's camera. This is how it works: To give the app a reference point for scale, you include a credit card (or equivalent wallet-sized card) alongside the object you're measuring and then take a photo. Every credit card is the exact same size, so it's a clever way to provide a scale reference with something you probably have in your pocket. (And no, the photo never leaves your phone so it's not like they're spying on your card.)
Then you drag and drop measurement lines on the card and on the object you want to measure. It's not a super accurate solution, but it will give you a decent estimate. Be aware that it doesn't account for perspective distortion so it's only intended to measure objects on a flat surface. But it works pretty well -- the keyboard I was measuring is exactly 28.4cm long; the app estimated 27.7cm.
Because it uses a wallet-sized card as a reference, you can only measure fairly small objects, and the UI is a little confusing at first, but the developer will hopefully refine it. The limited utility certainly makes the $4.49 price questionable, but I enjoyed the novelty enough not to mind.