Adafruit ItsyBitsy 32u4 - 5V 16MHz (AF-3677) USB bootloader AVR109 compatible (same as Flora, Feather 32u4,
Upload STM32F103C8T6 USB Bootloader | Program STM32 via USB Port
STM32 Bootloader | Programming STM32F103C via USB Port - YouTube
AVRUSBBoot - USB bootloader for Atmel AVR controllers - fischl.de
USBtinyISP USB Tiny AVR ISP Programmer for Arduino Bootloader Meag2560 Uno R3 M | eBay
nRF52840 USB Key with TinyUF2 Bootloader - Bluetooth Low Energy [MDBT50Q-RX] : ID 5199 : $15.95 : Adafruit Industries, Unique & fun DIY electronics and kits