The STM32 found popularity with the Blue Pill form factor. With the help of stm32duino, they have support in the Arduino IDE. While these are already good, Jana Marie Hemsing took the STM32 a step further to make an even better board. Using an STM32F072, he created OtterPill, which is an STM32 with the similar form factor and pinout as the 8-bit Arduino Nano, along with a few other additions.
The STM32F072's core is an Arm Cortex-M0. The OtterPill's pinout shares the same pins as the Nano where possible. For example, the power and ground pins are an exact match, as well as specific purpose pins like I2C, UART, and SPI.
OtterPill has a few differences from the blue variant. First, OtterPill supports being re-programmed through a DFU mode, allowing the user to flash it with the utility dfu-util. Second, the board has a USB Power Delivery (USB-PD) physical interface chip (PHY). Access to the USB-PD PHY means it is possible to power the board, and your project, with up to 20 volts. Alternatively, if you do not need that kind of control, then you can leave the PHY chip off your build and use resistors to force a USB-C supply to provide 5 volts and up to 3 amps.
Speaking of populating a board, currently, you cannot buy OtterPill. You can, however, get all of the design files from Hemsing's OtterPill GitHub repository. Both the schematic and PCB are available as KiCad design files. You can also find (future) firmware examples and documentation there. Once you get the PCB made, the expected BOM cost for the board is in the range of $3-4. Not bad for a 32-bit board in the Nano form factor!