wasp-os - Firmware for smart watches based on the nRF52 family of microcontrollers (including Pine64 PineTime)
hackattic - No nonsense, real world programming challenges
InfiniTime - InfiniTime is an open-source, C++-based firmware OS specifically made for the PineTime smartwatch.
Codewars - Achieve code mastery through challenge.
watchOS - Apple's new OS for the Apple Watch ⌚
Real Dev - Real-life coding skill challenge