I never announced it here, but just wanted to let everyone know that there is another GPX mapping program out there, TopoFusion. We use GPX as our primary storage format. We're quite happy with the concept and specs in GPX; bravo to everyone.
I also wanted to let you know that I'm working on simulating recreational trail users on GPS tracks and am getting people involved collecting data in the GPX format. I'm really insisting on GPX because I need the time component with each data point.
Also, I will soon be extending GPX to include topology information. TopoFusion works with networks of trails and there is currently no way to store topological information in GPX. I will probably make them private extensions for now, but if anyone else is interested in helping (or interested in networks in general), I'd love to hear from you.
More information about all this can be found on our site, www.topofusion.com