Bitty Blue provides a collection of fun things to do with a BBC micro:bit (or compatible device) and Bluetooth. Play with 3D "PolySquiggles", use your micro:bit as a compass, have fun with the micro:bit's buttons, design patterns and send them or text to the micro:bit LED display, connect and control electronic circuits. Lots of fun and educational too!
This application is available for iOS and Android smartphones.
When you first power up your micro:bit after installing the bitty blue hex file, you will be prompted to "draw a circle" by a message which scrolls across the display. Hold your micro:bit so that the display is oriented in the vertical plane and then slowly rotate it through 360 degrees. You should see a single LED pixel lit and it should move to the bottom of the display with respect to gravity. This strange process involves collecting both data about magnetic fields in the local environment and motion and results in your micro:bit being properly calibrated for use in that environment.