One thing you should always carry with you while you're riding is a multi-tool.  Nothing is worse than breaking down on the side of the trail and not being able to fix it.  The Crankbrothers M5 tool is the perfect trailside companion with it's compact size and 5 essential tools.  These high-tensile steel multi-tools feature a precise fit and finish and side grips for a secure handhold even with gloves on.  It comes with 3, 4, 5, and 6mm hex wrenches and a #2 Phillips screwdriver.