What is the best and widest used open source board? (Except Arduino UNO)

[Scrts]

Just FYI: http://linuxgizmos.com/catalog-of-122-open-spec-linux-hacker-boards/

[fki82]

The price to performance ratio is quite good on the blue pill STM32F103 boards. No 32bit Arduino clone is nearly as cheap.
It has been around for a while, so there is plenty of information online.
https://wiki.stm32duino.com/index.php?title=Blue_Pill

[dave3533]

Adafruit, Sparkfun, or Digilent would be the first few places I'd look for something like that. I'm not sure the Parallax Propeller is opensource or not, but that's also an interesting choice.

[robertferanec]

The price to performance ratio is quite good on the blue pill STM32F103 boards.

- I will get the blue pill board. Many people recommend it.

[wilfred]

STM32 "Bluepill" - My favorite at the moment

I like it. The chip looks good. A lot of pins, direct USB (no converter needed), 5V tolerant pins. Together with Arduino Nano, one of my favorites at this moment. Thanks for the tip.

What about the Arduino Leonardo.
https://www.arduino.cc/en/Main/Arduino_BoardLeonardo

[robertferanec]

What about the Arduino Leonardo.
https://www.arduino.cc/en/Main/Arduino_BoardLeonardo

Leonardo is very nice, just too similar to Uno. I have already made a modification of Uno (https://www.28pins.com/) ... but maybe this could be also a way to go (Leonardo has some advantages).

PS: I had a look also at Arduino MKR Vidor 4000 - software and community for this board is not ready yet. Currently I am playing with Blueduino https://wiki.aprbrother.com/en/BlueDuino_rev2.html

[westfw]

What about one of the SAMD boards?  Adafruit and Sparkfun both have a suite of OSHW based on SAMD21, and they (can) use the Arduino IDE.
"Blue Pill" is more of a "manufacturer reference design" than OSHW.  I'm not sure that I understand the differences, legally, but I don't know if you'll find a schematic and PCB floating around with a big "This designed is OSHW" license disclaimer.
There's the "Wiring" board, which is essentially the Arduino predecessor.  It faded because it was originally "too expensive", but ... costs have changed and it might be worth reviving.  Extra pins!  (not "popular", but as an Arduino compatible...)
https://hackaday.io/project/62511-revive-wiring