This is a simple belt tensioner made by 3d0g.
I've made the following changes:
Reworked slot and teeth to fit GT2 2mm pitch belt.
Added a third screw to clamp tighter around on belt.
Thickened sides a bit around nut.
Rounded the edges to the slot to make it easier to insert belt
When tightening, I found it kept wanting to pivot and not pull away from the printer body. I addressed that by adding nuts to the end of the screws for a more stable "foot" and pulling the loose cable end up and over, fastening it to the printer with a cable tie.
This pivoting inspired me to design this: http://www.thingiverse.com/thing:82388