This mount goes between the x-carriage and extruder body to hold a fan at 45degrees to the hot end. It was designed to fit the Printrbot, but may fit other printers too. It holds a 50mm diameter fan.
The benefits of using this are twofold. Firstly, each layer of a print cools faster, enabling higher quality at faster speeds and on smaller objects. Secondly, it helps cool down the hotend so that it is less likely to warp the extruder body.
I made the mount quite thick at 6mm to raise the hotend up a little and give more z space. On my Printrbot up to 10mm of extra z space could be gained by raising the hotend.
Note, this style fan mount should only be used with insulated hotends, otherwise the hotend will cool down too much and perform poorly.