GPS software for my data

Hi,

I've been looking at all kinds of GPS software for mobile devices, laptops, etcetera, but all of them seem to only work with their own, included maps. None have map data for my part of the world, but I have my own files -- shapefile format, but I can convert to any other kind of GIS file.

So, open source, proprietary, whatever -- is there any GPS software, with navigation, that isn't really software plus map?

Garmin Mobile PC works with non-Garmin, 3rd party, maps. :)