These amazing looking smart juggling balls are intended to be used as an educational tool, by interactively demonstrating physics, but they look so good, they could be used by street performers at night.
Each ball features a Pro Mini, along with a 6 degrees-of-freedom sensor, some LEDs and a transmitter. The sensor allows the balls to change colour at different times of the throw, depending on acceleration, rotation and gravity, making a beautiful hypnotic pattern of light. These balls can also be controlled remotely, which allows the mode to be switched during a performance, possibly to flashing, or different colour schemes.
Arkadi has open sourced the whole project, including the code, schematics and instructions, which you can grab from GitHub. As the design is educational, he intends to create content to coincide with the project, so that it could be used in school classes.