PineTime: a smartwatch for open-source software
The PineTime is an inexpensive smartwatch developed by PINE64 that is designed to run open-source operating systems. Despite its low cost, however, it has most of the features expected from more expensive, proprietary smartwatches. Because it runs open-source software, though, interested developers can add any other useful features that they dream up.
Users can flash any compatible operating system to the PineTime. Currently, there are two major projects: InfiniTime, which comes preinstalled and is written in C++, and Wasp-os, using MicroPython. InfiniTime is the more popular of the two, perhaps in part because it comes on the watch. But, perhaps surprisingly, the watch ships with an outdated version of InfiniTime; users should upgrade in order to benefit from several improvements, including power optimizations that can double the watch's battery life. There are two other firmware projects that I know of out there: GopherWatch, written in Go and currently in early development, and a seemingly unnamed and currently unmaintained firmware written in Rust. I did not test either of those.