Kehribar added a UART bootloader to his Little Wire AVR multi-tool. The Little Wire is an AVR programmer that can also be used as an SPI bridge, analog-to-digital converter, and PWM generator.
This project uses the ATtiny Reset pin, which means a high voltage programmer has to be used to setup the pin configuration fuses for each upgrade. This was a design limitation Kehribar didn’t like, so he added this fast tiny & mega UART bootloader (AVR Freaks requires log in) .
Via the forum.