The Slashdot forums have an interesting discussion currently happening regarding hackable GPS units. There’s also a fair amount of chatter about beating people with keyboards and why mother-in-laws being driven off cliffs by faulty GPS devices is a good thing (if you’re looking for a laugh). But there are some good resources for those looking to modify their navigators.
Really, anything that stores all of its files on an SD card is the easiest to modify because you just have to replace the files. There isn’t any hacking or coding involved. GPS navigators such as the Navigon 2100 would be good examples. I also learned that there is quite a large Pioneer AVIC-hacking community at http://www.avic411.com and the open source Openmoko Freerunner is a popular choice for GPS hackers as well.
Have you ever modified a GPS navigator? A TomTom or Garmin? Please share!