Charles is a really useful http debugger for when firebug's http logs (or flashbuilder 4's now) don't provide enough clues. I used a free tool called Fiddler before, but Charles is so much easier/better. And its creator is a fellow Aucklander!

Flashdevelop is an amazing actionscript/flex/php/haxe (and more) IDE. It still has (IMO) better actionscript code completion than other actionscript editors I have used. I have used FlashDevelop for coding all my contributions to Degrafa, and as my long term introduction to beginning work with Flex before moving to Flex/Flash Builder.

Keepass is a great (and free) password manager utility. It lets you have 'one password to rule them all and in the darkness bind them".

I use rapidsvn as a supplemental svn client outside of flex/flashbuilder/eclipse. I tried tortoisesvn at one point and went back to rapidsvn as a personal preference.

Wampserver is a clean and easy way to install and maintain an AMP stack (apache, mysql, php)  on my windows machine, and it comes with a nice little system tray app that makes my life so much easier for managing the overall setup. Highly recommended for Windows users.