Finally, a fan that really BLOWS! I wanted a better option than the raft of 40mm boxer fan type cooling arrangements. Still a little bit of a work in progress (in that I have been using a previous version of this fan on my RepRapPro Mendel for the past few months, these files are the improved version)
UPDATE 28 July 2014:
Printed and tested. Definitely some trimming and fitting required to get the parts to fit properly. I put a small (2mm) spot of superglue on the motor plug to keep it in place, and a similarly sized spot on the tip of the motor shaft before sliding the impeller on. You may have to add some packing between the motor housing and the fan housing to get the fan to spin without hitting the sides.

UPDATE 17 July 2014:
I haven't made any allowances in these models for die swell, so your slicing software needs to be well tuned up or they won't fit together. I printed version 2 of this with cura (REALLY fast, but not very tunable) and the parts didn't fit (haven't tried with slic3r yet), so I made a version with a bit of extra room. I have also changed things around in V2 so that the mount is less fragile and the motor housing can be printed without support.
The models are:

Motor housing, fan housing, motor plug - version 1 (no die swell allowance)

Motor hosing v2, Fan housing v2, Motor plug - version 2 (no die swell allowance)

Motor housing v2 eased, Fan housing v2 eased, Motor plug v2 eased - version 2 with die swell allowance

Impeller - the original version, can be used with all 3 variants

Impeller_serrated - an attempt to reduce fan noise.