Some industrious programmers have found a way to hack the Archos 605 WiFi portable video player to run the Qtopia Linux platform. By the looks of it, the Qtopia hack doesn't add much in the way of extra media features (the Archos does pretty well as-is), but it opens the door to developing the Archos 605 WiFi as a more generally useful and configurable tablet PC. The Qtopia hack appears to work on older models of the Archos players as well, although the Archos fifth-generation players seem to be easier to configure.
Personally, I think the Archos 605 WiFi is a cool enough gadget on its own, but there's always something to be said for hacking a gadget just to explore the possibilities. The video below should give you a better indication of whether the hack is worth voiding your warranty.