INTERNATIONAL

IMBA - International Mountain Bicycling Association: Write to IMBA P.O. Box 7578 Boulder, CO 80306. A nonprofit organization to promote environmentally sound & socially responsible mountain biking.
http://www.imba.com/

NATIONAL

National - League of American Bicyclists:The League of American Bicyclists promotes cycling for fun, fitness, and transportation and works through advocacy and education for a bicycle-friendly American
http://www.bikeleague.org/
National - The Outdoors Club: A community web site for outdoor enthusiasts to find activity partners and plan hiking, climbing, rafting, kayaking, biking or other outdoor activities. Membership is free and all events hosted by the club are non-profit.
http://www.outdoorsclub.org