The Elektor Raspberry Pi Ruler is a handy ruler with lots of practical information about the Raspberry Pi printed on it. With it you will always have the RPi’s most important mechanical and electrical specifications and often needed terminal commands handy.
But the ruler is more. Using this kit of parts you can solder 22 LEDs and resistors on it, connect a VL53L0X Time-of-Flight (ToF) sensor and plug it on a Raspberry Pi to turn it into an electronic ruler capable of measuring distances of up to two metres. Full details on how to do this can be found here.