Blinky is a Raspberry Pi headband (too small to be a HAT, so a headband, right?) with LEDs on all of the GPIO pins. Each LED has its own independent MOSFET driver and is supplied from the 5V rail of the Pi. The headband (sorry) also does not have any pull up or pull down resistors, meaning there is no loading or interference with any other uses of the GPIO pins (I2C, SPI, etc.). The 40 pins on the header are also all labeled with their functions for easy reference.
Why did you make it?
Over the course of many projects and protoytpe builds I found it useful to visually see the state of different buttons, switches, communication lines, etc. This has helped to quickly identify the source of a problem, whether it be a bad switch, wire or programming.