Arduino compatible chips

Does anyone know what AtMega chips are compatible with the Arduino bootloader? Ex. Atmega328, Atmega644, Atmega32A etc...

I just had the felling only Atmega168 or Atmega368 worked with Arduino, just confused.

As far as I understand it, each chip needs a custom tweak or revision of the bootloader code. There are timing divider variances, port location variances, etc., in each model. However, the basic bootloader source code started on the ATmega8, then the ATmega168, then the ATmega328, and now the ATmega1280 for the "Mega." Improvements and functionality expansions have been going on as well. I doubt you could fit today's bootloader code on the 8 anymore, and you can't just copy them back and forth.