For road and mountain bikes, it's tough to beat Easton's EC70 unidirectional carbon stem. This carbon wonder keeps the weight down and offers great strength for awesome power transfer as well as vibration damping for all-day comfort. Plus, the replaceable hardware means no more stripped bolts and long-lasting function. And it's compatible with carbon handlebars, too.
Stem length is measured along the side of the stem from the center of the steerer to the center of the handlebar clamp (see diagram). Keep in mind that as a stem provides more rise, its effective length decreases. In the example above, if the +17-degree stem measures 135 mm, it actually extends 108 mm parallel to the ground.