NEW: see the PLA Cooling Challenge http://www.thingiverse.com/thing:627590
PLA layers need to cool before the next layer is printed. If the layer is small (that is, doesn't take too long to print), it may still be warm and soft when the next one is printed which may cause visible deformation and reduces print quality. The problem can be mitigated to some extend by slowing the printing speed or using a material cooling fan but is still significant with very small layers.
This thing is a dummy object that you can add in your slicer to add delays between consecutive layers of your real object. Just ''place one or more far from the actual object and print as usual.
The cooler was designed to consume minimal amount of material and can be customized using the Customizer button on the right.