BICYCLE HELMET SAFETY INSTITUTE - An extremely interesting site where you can find detailed information about bicycle helmets, statistics about their use and a list of interesting related web sites. And remember... always wear a helmet, your head is really too important!
INTERNATIONAL BICYCLE FUND - IBF is a non-profit organization dealing with bicycles from many different point of views: transport, tourism, safety and many others. The site is very wide and well done even if for the MTB fan the most attracting section is Bicycling in Africa, a complete guide for organizing an MTB travel there. The site is simple with limited graphics, but fast and intuitive to browse.
IMBA - The International Mountain Bicycling Association.
NORBA - The National Off-Road Bicycle Association.
U.C.I. - It's the official site of the International Cycling Union. Part of it deals with MTB from a general race-oriented point of view. You can find here useful info such us data on the history of MTB, up-to-date U.C.I. ranking, race calendar with plenty of information. The site is modern and attracting with very nice graphics and it's also fast and easy to browse.