GpsDrive is a free GPS navigation system which works on GNU/Linux and several BSD's including Mac OSX. It is free software licensed under the GPL. The main website is at http://www.gpsdrive.de.
GpsDrive can render OSM data on the fly as you move along the road. Use osm2pgsql to load a planet.osm file into a PostGIS database and make sure that GpsDrive was built with Mapnik support. In the 'Map Control' window enable 'Mapnik mode' and you're away.
If you don't want to go to the trouble of setting up a PostGIS database, you can also download Tiles@Home, OpenCycleMap, and NASA LANDSAT backdrops for your current location, or by using an included script along a chosen route or prior GPX track.