A simple pulley, but carefully dimensioned for a perfect fit around a 623ZZ bearing and inside a P3Steel / Prusa i3 X-axis GT2 belt tensioner.
I am using it (as shown in the pictures above) as the X-axis belt tensioner pulley for my set of improved Prusa i3 / P3Steel parts.
I found it practical to insert the 623ZZ bearings in the printed pulleys while they are still warm on the heatbed.
Another tip is to slice it in Slic3r (great piece of software!) and set the Seam Position parameter to Random, this avoids creating a seam on the perimeter of the pulley.
OpenSCAD sources are included so the design can be further remixed.
Printed in PLA with 40% infill, 0.2mm layer height.