I rolled my own STM32 breakout board because reasons. I'm using the STM32F070 processor (ARM Cortex-M0), as it has built-in USB device support. The idea was to create a reference design different from the Nucleo, Discovery, and Blue Pill boards. It has been a great learning tool while working through Carmine Noviello's Mastering STM32 book (https://leanpub.com/mastering-stm32).
I added pins on the back for an ST-LINK/V2 tool, and the cheap programmer/debugger (like this one from Adafruit: https://www.adafruit.com/product/2548) has proven extremely useful for in-circuit debugging.
I love the Nucleo, Discovery, and Blue Pill boards, but I find most of them lacking as a reference design. The Nucleo boards have a built-in debugger, which is fantastic, but I found it difficult to go from ST's documented schematic to my own design. They seem to be mostly a kitchen sink type of development board (add all the features!) where discerning only the necessary parts took a lot of time and cross-referencing other manuals.
The Blue Pill does not have built-in USB support (it looks to be bit-banged from the STM32F103), and one of my goals is to provide a bootloader via VCP or uf2. While some of the Nucleo boards have built-in USB, most are just broken out to pins, as the USB connector is reserved for the ST-LINK chip.
As a result, I've put together my own breakout board for one of the newer STM32 parts based on the ARM Cortex-M0 processor: the STM32F070. I had v01 manufactured by OSH Park, and everything seems to be working (so far). I have a few minor fixes to the layout, and I'm hoping to get the PCB manufactured with white solder mask next time.
While I was able to put one together via hand soldering, it's not something I want to do again in the future. As a result, this project is also giving me an excuse to invest in a reflow toaster oven for "research purposes."