We’re currently testing a new fan duct for the TAZ. The fan duct directs air at the tip of the extruder, cooling the filament just as it’s being laid down. While this can wreak havoc on an ABS print, it can greatly improve the quality of prints with bridges and overhangs especially in PLA.
You can control the fan through slic3r, using the cooling options under the filament tab. We’re working on a guide to printing with a fan for cooling, but for the meantime you can find the stl and g-code here (fan mount ss-3): http://devel.lulzbot.com/TAZ/Guava/production_parts/test_parts/fan_duct/