Angry Asian: The definitive guide to maintaining and tuning your mountain bike suspension fork.
Biking Articles by Sheldon Brown and Others: This site contains a wealth of technical articles about bicycle maintenance for both mountain and road.
Chain-suck in a Nutshell: Jonanthan Levy details the causes and mechanisms of chain suck and how to avoid and prevent it.
Chainffect: Discusses technical aspects of suspension systems on mountain bicycles, with technical illustrations.
Mountain bike mailing list faq: Extensive FAQ compiled and appended by members of a mountain bike mailing list.
Singlespeed Mountain Bikes: Many cyclists are rebelling against the excessive complication, fragility and weight of current mountain bikes. Discusses move to using simple, one-speed bicycles.
The cycle shed: Single speed, wheel building, t-shirt printing, yorkshire (UK) based stuff.