Here are some of my Printrbot Simple mods that I think are essential upgrades!
The filament guide is a derivative of thing:128240 by johnbiehler. I added a feature on top to attach a 1/4" PTFE tube (from Lowe's) to guide the filament in better from the spool on the side (thing:118559).
The fan mount is my own design for a 40 mm fan. I got my fan from Radioshack. The Z coupling is also my design.
The feet are thing:129448 which I attached some rubber feet to the bottom of to prevent slipping.
Printed everything at .2 mm layer height. The filament guide should be printed with the clip part facing down and support material. The fan bracket needs support as well. The coupling I printed hollow with 3 perimeters.