Optiboot

Its predecessor ate two kilobytes of an ATmega328P's 32. Optiboot does the same job in a quarter of that and handed 1.5 KB back to the user's sketch, which was enough to make it the default on the Arduino Uno from 2010. It speaks a subset of Atmel's STK500 protocol at 115,200 baud.

The trick that makes it convenient lives off-chip: the USB-serial chip's DTR line is wired to reset through a capacitor, so opening the port reboots the board. Optiboot waits briefly for data and otherwise jumps to the application. It also checks the reset cause — a watchdog reset goes straight to the application, so firmware that resets itself on purpose doesn't sit around waiting for an upload.