We already knew that hacking of Archos 5 series players was picking up some considerable steam, but it now looks like that effort has produced the first genuinely useful (to some) hack, with the Linux-based Qtopia platform now up and running on at least some of the devices. Better yet, there's already a complete how-to available to guide you through the process, although, as always, it's not exactly suited to those concerned with warranties and whatnot. What's more, while the hack is intended primarily for 5 series devices like the Archos 605, some users have apparently also had success with the 604 WiFi (as seen above), although that seems to be a slightly more complicated process. If you're still not deterred, you can hit up the links below for the full rundown and relevant discussion.
Buy from Amazon - Archos 605 Wi-Fi 160 GB