I really like the STM32 family of ARM microcontrollers. This breakout board is a narrow design that fits perfectly on a breadboard. Every pin is accessible. It has USB, a reset button, and a bootload button. The chip itself has a permanent USB bootloader that is activated by the bootload button. This means that no equipment is required to program this microcontroller, no expensive debugger, nothing at all except a USB cable.
Design files are open source and can be downloaded, there is also a short video about the bootloader and more instructions
I am also giving away blank PCBs to those interested (see “giveaway” on my page), Seeed makes more than I need, every single time
Via the forum.