There are lots of DIY watch projects on the internet. Some of them are great, some of them lack to feel of finished product. If you looking something like this check out at F*watch project which open source and built by using free tools. It was developed at CERN as a gift for retiring colleague. It has everything you would expect on modern digital watch. It has been stuffed with bunch of sensors including GPS, Pressure sensor, accelerometer, compass, ambient light sensor. Watch has 128×128 LCD display with back-light, micro USB connector, four push buttons buzzer, vibrating motor, and microSD memory card. It is powered by 500mAh Lithium-ion battery.
To make it all possible, schematic was designed on four layer PCB with KiCad. Case was 3D printed out of resin. Plans were developed with open FreeCAD design tools. Clock is ticking around ARM Coretex-M3 microcontroller from SiLabs Giant Gecko featuring 128KB RAM and 1MB of Flash. Watch software runs a FreeRTOS operating system. Watch is fully functioning already, but seems that team are open for bug reports and feature request.