GPX Files sources
"Multi Countries" sources
Bikely - www.bikely.com
An amazing web site: a Google Maps mashup shows you the routes on map. There is also a very good search engine to specify which kind of track you are looking for.
Bike Radar - www.bikeradar.com
The same as Bikely.
UNITED STATES
GPS Mountain Bike Trail Guide - Northern California
Crankfire - Connecticut Mountain Biking
GPS Trail Maps - Trails in the Western US
TrailCentral - Mountain Bike maps of Colorado
Team TopoGrafix - Massachusetts mountain biking trails
NETHERLANDS
CANADA
Trail Repository - Canada trails
ITALY
Gemini MTB - Italian mountain biking on Elba
GERMANY
GPS Tour.info - mountain biking in Germany and surroundings
BBBike - German route-planner for cyclists in Berlin
FRANCE
Igor's Blog - Découvrez les alentours de Perpignan en Mountainbike
Les Randoactifs - French mountain biking trails
UNITED KINGDOM
Scotland - Jeremy's Mountain Bike Page
If you know a good source for bike related GPX files, please let me know about it send me a mail