Here are some of the plugins that I always install when using Flash Builder. Let me know what you think, and let me know what other plugins you recommend.
quantumDB – (quantum.sourceforge.net) – this will give you access to your MySQL database from within Flash Builder. Very useful to check what is going on and also to check if your SQL commands return what you think.
phpeclipse (www.phpeclipse.com) – this will install a php editor and internal web browser, so you now do all your php editing from within Flash Builder.
