My wishlist for a CSS inspector has had been already fulfilled: http://code.google.com/p/fxspy

A great code formatter as Eclipse plugin for Flex Builder: http://code.google.com/p/flexformatter
Download: http://sourceforge.net/projects/flexformatter/

HP has published a security analysis tool for Flash content. It´s also a decompiler to inspect the Actionscript code: SWFScan

Good overview about security issues in Flash:
http://www.owasp.org/index.php/Category:OWASP_Flash_Security_Project
http://www.adobe.com/devnet/flashplayer/articles/secure_swf_apps.html

A great resource for open source flash/flex tools and projects: http://www.flashchemist.com/104-free-opensource-apis-libraries-and-tools-for-the-flash-platform.html

At styling and changing my blog theme, i really fall in love with Web Developers (Firefox addon) CSS tools.
Just rollover or select an element and see the css used. Edit it live and see immediately the changes.
How nice would it be to have something inside Flex Builder?
For re-styling or adopting Flex projects this would be a great helper-tool, specially if you edit custom components from other developers.

If you feel this would be a nice feature, vote for it on the Adobe Bugbase.

You probably have seen already the Flex Style Explorer. It gives a good overview of the different styles of the standard Flex components.

BTW: Did you know that parts of Flex Builder are open source (the derived stuff from Eclipse)? You can download it at Adobe.