There are several complete and free AVR boot-loaders around internet, but this one can be great choice as it has nice and intuitive graphical interface. It seems that it is tested on very few AVR’s, but should work on other too.
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Boot program for chip is written using codeVisionAVR where free evaluation copy should be enough to compile. And another part is a Windows (works on Win 7) application that is available as Academic Free License. Bootloader itself is capable in programming Flash and EEPROM files and talks via COM port.