Today I've released the beta-1 version of picoboot. Requiring only 66 bytes of flash, picoboot is the smallest AVR bootloader, taking a quarter of the space of other "tiny" bootloaders which start at around 512 bytes. Not only is it the smallest available bootloader, it is the smallest possible bootloader for AVRs with a 64-byte page size such as the ATtiny84. Picoboot is also fast, taking less than 3 seconds to write 8126 bytes to flash.
Future plans include builds for the ATtiny2313a and support for zero-wire auto-reset.
The code is open-source, and available here: