F*watch! Why should your watch be different?
(FOSS) F*watch is an infinitely hackable GPS watch with many sensors based on a 100% Free design. Everything is free, from the PCB and watch housing design to the software stack. Moreover, only free software FOSS tools have been used during the development.
The FOSDEM2015 talk described the development process and shows a first prototype, along with performance measurements and future plans.
Links: F*Watch wiki