Using GPS tracks with NASA World Wind
If you have Garmin tracks (or for that matter any GPS tracks) you can convert them and show them as an overlay on NASA World Wind.
If you haven't heard of World Wind then go here http://worldwind.arc.nasa.gov/
Steps
Download "G7ToWin" http://www.gpsinformation.org/ronh/g7towin.htm
Convert the .gdb file to a .plt file ("Open" then "Save As")
Download "PLT to NASA World Wind" http://worldwind.czart.net/
Then convert the .plt file.
When you next start World Wind your track is available as an overlay.
If you can't be bothered to convert one yourself to see how it looks then the trip overlay for Spain 2005 can be found here
Routes in Spain
Routes in France
Close up of part of Spain 2005 showing 3D effect