Atmel AVR Disassembler

You can also use avr-objdump, a tool part of the avr-gcc toolset ( http://www.nongnu.org/avr-libc/ ). Ex:

avr-objdump -s -m <avr architecture> .d program.hex > program.dump

where <avr architecture> is found on http://www.nongnu.org/avr-libc/user-manual/using_tools.html

[plug]IDA Pro supports AVR disassembly[/plug]:

As for opensource, AVR GCC package includes a port of objdump, including disassembling functionality.