Thanks to the fact that Archos used GPL and LGPL code in constructing their firmware for the Archos 604 and 604 Wi-Fi devices, they're forced to release just about the source code for said firmware into the public domain. What does this mean to the average Archos 604 user? Not a whole lot, at least right now. The code is pretty much complete, but lacks a few things like signatures, and some non-GPL utilities Archos used to build, link, and generate the final firmware files.
But, if the open source community can find out what goes into these gaps and plug'm, soon enough you should be rocking out to an open source Archos capable of playing back weird codecs, streaming things to other things, and doing all kinds of stuff that it wasn't meant to do originally&madsh;similar to the way that open source users took the Linksys router and beefed it up.
Download Page [Archos - Thanks Rick!]