What is it? It's a copy of http://www.engbedded.com/cgi-bin/fcx.cgi, but written completely by me, and done with a mixture of PHP, Javascript, and C#. I am absolutely aware of how blatant of a rip-off my version is, but my philosophy has always been to write my own tools if I don't like existing ones.
Why copy it? How is my version different? I HATE how the Engbedded's version uses server side requests to perform the calculations, this meant that the page refreshes whenever you make a change. The URL for Engbedded's version is very long when you use it. My version is also aware of possible fuse bit combinations that may have a behaviour that is not listed in the Atmel part descriptor XML.
How did I do it? The C# program I wrote generates the HTML elements for each chip using the XML part description files provided by Atmel. The PHP page takes the GET request from the URL and fetches the correct calculator HTML page, and also adds in a script that sets the saved fuse values upon loading the page if the URL contains saved fuse values. The javascript does bitwise calculations to match presets with fuse settings, and manipulate the page elements.
Some of the chips may not have good descriptor files, thus the page may be useless for that chip, this mainly applies to the new XMEGA chips.
Read more about it on the actual calculator page.