I have a sony clie with a serial GPS attached. I recently discovered that I could record tracks using cetusGPS and upload them to www.gpsvisualizer.com to get the tracks mapped out on overlays. It supports things like coloring based on altitude, speed, distance, overlaying onto different types of map sources, and even google maps and google earth.
Here's a test I just did the other day. I turned the track on as I was driving southbound on I-93 in Cambridge, MA and turned it off once I found a parking spot at MIT. You can see the spot where it lost communication in a short tunnel on Storrow Drive and cut off the corner when drawing it. You can even see when I changed lanes and some strange glitches when I tried to parallel park in spots I wasn't going to fit in, in front of hydrants/crosswalks/yellow curbs/etc. Very neat!
Google Map (zoom in/out if the track doesn't immediately appear)
Has anyone tried anything like this for trail riding?