TOP #6  OPEN SOURCE HACKABLE SMARTWATCH

#1 Bangle.js 2

GPS, Compass, Accelerometer, Altimeter, Heart Rate, Pedometer, Touch

Programmable Using JavaScript

#2 PineTime

Any open-source operating systems  built on top of numerous RTOSes

#3 Watchy

Powered by an ESP32 with e-paper display Program via Arduino

#4 Open-SmartWatch

Powered by  ESP32-pico-D4 with BMA400 Accelerometer + Pedometer

#5 CulBox

Not Available anymore 

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#6 OpenHAK

Using the Simblee 32bit MCU module. Fully Arduino compatible.