This was actually mentioned previously by someone else in a thread on micro-payment systems, but i've come across it a couple of times so thought it deserved it's own post:
PPCalc.com is a free web-based PayPal fee calculator. It is an online tool that allows PayPal users to quickly calculate their PayPal fees before accepting a payment, or reverse calculate their fees to determine what amount a person must send them for them to receive a certain amount after fees. It supports calculating PayPal fees between different currencies and includes daily-updated currency exchange rates and support for cross-border transactions. It also supports PayPal's new Micropayments (digital goods) rate, and the new rate Personal accounts are charged for accepting limited credit card payments. It was originally written on May 13, 2005 and has been updated numerous times since to account for changes in PayPal fee rates. Currently, it supports calculating PayPal fees for 103 different countries and 16 different currencies and includes a built-in currency converter.
PPCalc.com is simply a third-party tool and was not written by, nor has any official connection with PayPal.com or eBay.com.