Mike Rankin created this tiny sensor board with an ARM Cortex M0+ microcontroller and OLED display:
It’s pretty tiny so a 4 layer board made the whole job easier. The top and bottom are pretty much dedicated for components, layer1 are tracks and power traces, layer2 is a ground plane. Mixing up an internal plane and routing layer was interesting.
Atmel SAMD21G Sensor Board – [Link]