Razorblatter, you are unclear. Chas, you too
TomTom and Garmin are both Linux based devices and can only run their own GPS program and no other.
Basically, if the device is Windows CE based, any GPS program that is developed for CE can run on that device. And even TomTom and Garmin have Win CE builds that can run on almost any device.
I can not say about Navigon. Though I know Navigon has CE builds, I do not know if Navigon devices are all CE based or not. I do know that (on some devices?) it's needed to dive deep in the program code to get Navigon completely working on a non-Navigon device
There are other limitations too. TomTom and CE6 are no friends. Garmin XT is ... aged. Some devices need unlocking before you can do anything with theM. Some devices don't need any unlock at all. And unlocks depend on the device. Or you can run any program on it but lack a hardware driver and don't have TMC (e.g).
As an example that has come forward lately, Medion devices are easy to unlock. But only the older Medion devices are TMC capable with other than Gopal software. But iGO (on low memory Medions) and Primo run fantastic (with tha lack of TMC on the newer ones). Navigon needs often a program code intervention on (certain?) Medion.
I hope this makes some sense.