Well U2U-AE is a simple USB to UART converter that allow you to program AVR and ESP MCUs really easily, for AVR you just need GND, RX and TX lines... sometimes DTR too (for auto reset). For ESP8266 ...

This is a PlatformIO based Arduino project that turns a supported microcontroller into a UART to USB HID Keyboard Bridge. When connected to a PC, the device identifies itself as a USB HID keyboard.