GT2 Timing Belt
GT2 timing belts and compatible pulleys are popular choices for RepRaps and RepStraps -- see Choosing Belts and Pulleys for alternatives.
The GT2 timing belt is a possible replacement for T2.5 and T5 timing belts that some believe may cause backlash. The GT2 series of belts are designed specifically for linear motion. They use a rounded tooth profile, with 2mm pitch, that guarantees that the belt tooth fits smoothly and accurately in the pulley groove, so when you reverse the pulley direction, there is no room for the belt to move in the groove.