The new Arduino prototyping kit allows you to build and program your own circuits using the fully functional and production-ready Arduino prototyping board.

Normally, an AVR microcontroller can’t write to its own flash memory except when it’s in bootloader mode, and you’re stuck using EEPROM when you want to save non-volatile data.

With just enough memory to back up the AVR’s flash, the bootloader can shuffle the existing program out to the EEPROM while flashing the new program in. For more details, read the source.