The Jupiter Nano is an open-source hardware based on an awesome MPU product from Microchip called the SAMA5D27C-LD1G. The boards exposes a lot of IO in spite of a tiny form-factor, boots a full-fledged linux distro as well as can support microcontroller-like development flow.
Like the RPi Zero? You will love the Jupiter Nano!
I recently came across a wonderful, tiny, fully open-source board on CrowdSupply and loved it. Here's my take on this!