This is a parametric customizable fan spacer. 

I had an 80mm case fan that wasn't spinning because it was rubbing against the case. I designed and printed this in order to try spacing it off a bit. 

The other reason you might want a spacer would be to cut down on noise if the blades are too close to a perforated case.

In my case, it turns out the fan had died. I'll keep using the spacer with the replacement though. 

The stl is for 80mm, but you can easily measure and use it for smaller or larger fans. The OpenSCAD is customizable.

edit: added 120mm model with 5mm thick and 2mm thick, since this is perhaps more common