The Nokia N800 would make an awesome multimedia car audio device. Depending on how involved you want to get with re-purposing the device, you can do something like this:
Purchase some SDHC cards (looks like you can put up two 32 GB cards!), post a teardown on iFixit of it and take out the screen from the rest of the device, and fashion a custom display case for your car. Then, tuck away the rest of the hardware internals into your dash and route the appropriate wiring to your stereo head unit, which hopefully has Audio-In or AUX-in jacks. The installation is a bit more complex than that, but you get the idea.
With this new-fangled device you'll be able to play MP3s directly from your stereo, use it to make free teleconference phone calls via Skype wherever you find Wi-Fi (great for emergencies or to save on minutes), use the webcam feature to monitor entry and egress from your car (great if the car's stolen or if you need to blackmail family/roommates) -- the possibilities are endless.
Another user mentioned using it as a GPS, but I'm not educated enough with the device to know for sure if you can do that. It doesn't seem to include GPS capability from what I see on Wikipedia, but I may be wrong. That would certainly be another incentive for using it in the car.