Sizes (www.sizes.com) is an amazing free fact-packed website. Think of it as Wikipedia for everything related to figures and sizes.
Want to know the size of a Queen-size bed as opposed to a King size? Or how many volts they use in the electricity supply in Kenya? Or the markings that indicate the ratings of an electronic resistor? Or maybe the dimensions of a screw thread? The answers to these, and millions of other questions, are here to be discovered.
A hugely useful resource that's definitely worth bookmarking.