Y axis belt tensioner for Prusa i3. Needs two 624Z (13x5 mm) bearings and two M4x25 screws. Preferably use a nyloc nut for the tension screw.
Update: Version 2 (the v2 files), slightly longer for allowing a pulley. Only needs one 624Z bearing. The washers aren't necessary. I actually changed them for a couple of regular M4 washers.