Here's a few of my favorite links:

North Shore Mountain Biking (NSMB)
100% Canadian.  But hey, that's where some of the best riding is.  A ton of users, busy forums and a lot of contributors.

PinkBike
Also Canadian.  Less visitors, but these guys are always on top of what's up with the Freeride industry.

MTBR.com
Wanna-be American Mountain Biking site.  Good classified section for high quality bikes.

Ridemonkey.com
Great mountain biking forums.