http://www.keenpeople.com/index.php?option=com_downloads&Itemid=50&func=selectcat&cat=3 Maps of Europe, Illinois, Canary Islands.
http://www.bicimapas.com.mx/English.htm Maps of mountain bike trails in Mexico.
http://www.treknow.com/free/ Bike trails in North America. Includes Grand Canyon, and Lewis and Clark trail.
topofusion gps free maps More mountain bike trails, in Utah and Arizona.
If you are not satisfied with the free GPS maps (or with paid maps), you can actually make your own. Some people find it fun to do this and share the maps with others. Here are a couple of tutorials on how to make your own maps:
http://www.keenpeople.com/index.php?option=articles&task=viewarticle&artid=4&Itemid=3
http://www.gpsinformation.org/adamnewham/article1/gpsmapper.htm