It's so awesome!
It uses Microsoft Visual Studio (free) and it makes creating addons easier than WoW UI Designer!
It even has options to create the addon to Ace. And a lot of easy options.
http://www.codeplex.com/WarcraftAddOnStudio
Download Instructions:
1. Install .zip at the "Releases" Tab.
2. Open folder, open
setup.exe
2. A. if you don't have Visual Studios, it will install that, which will take 20min or so. then restart computer (go to step 2. B.)
2. B. after you installed Visual Studios, you open
setup.exe again, and install the WoW Addon part of it
Note: If you have Visual Studios, opening
setup.exe will just download the WoW Addon part.