If you are an Etsy seller you must know about this one! Rolbe.com has a free web-based tool can be used to easily calculate Etsy fees (and optionally PayPal fees) for any amount.
It also supports multiple quantity listings and has a special field for your "cost of materials". Now you can quickly determine your what your Etsy fees would be and how much profit you can make from selling an item on Etsy.com. He also has calculators for Amazon and Ebay!