When developing Flex and/or AIR applications there are a handful of tools that really make the difference between a good project and a great project. They are all very valuable and provide stability and clarity for the RIA development process; or at least they have for me. From Unit Testing to SVN utilities to frameworks and libraries I’ve found the following ten items to be very useful and helpful. I’d even go so far as to say these are critical for quality Flex RIA development; but as Les says: “It’s just a matter of opinion.” 
Cairngorm
Cairngorm is the lightweight micro-architecture for Rich Internet Applications built in Flex or AIR. A collaboration of recognized design patterns, Cairngorm exemplifies and encourages best-practices for RIA development advocated by Adobe Consulting, encourages best-practice leverage of the underlying Flex framework, while making it easier for medium to large teams of software engineers deliver medium to large scale, mission-critical Rich Internet Applications.
http://opensource.adobe.com/wiki/display/cairngorm/Cairngorm
Download: http://download.macromedia.com/pub/opensource/cairngorm/cairngorm2_2_1-bin.zip
Flex Unit
FlexUnit is a unit testing framework for Flex and ActionScript 3.0 applications and libraries. It mimics the functionality of JUnit, a Java unit testing framework, and comes with a graphical test runner.
http://opensource.adobe.com/wiki/display/flexunit/FlexUnit
Download: http://download.macromedia.com/pub/opensource/flexunit/flexunit0_9.zip
Subclipse
Subclipse is an Eclipse Team Provider plug-in providing support for Subversion within the Eclipse IDE. The software is released under the Eclipse Public License (EPL) 1.0 open source license.
http://subclipse.tigris.org/
Eclipse update site URL: http://subclipse.tigris.org/update_1.6.x
Eclipse
Eclipse is a multi-language software development platform comprising an IDE and a plug-in system to extend it. It is written primarily in Java and is used to develop applications in this language and, by means of the various plug-ins, in other languages as well - C, C++, COBOL, Python, Perl, PHP and more.
http://www.eclipse.org
Download: http://www.eclipse.org/downloads/
ASDoc
ASDoc is a command-line tool that you can use to create API language reference documentation as HTML pages from the classes in your Adobe® Flex® application. The Adobe Flex team uses the ASDoc tool to generate the Adobe Flex Language Reference.
http://livedocs.adobe.com/flex/3/html/help.html?content=asdoc_1.html
Flex Ant Tasks
The Adobe® Flex® Ant tasks provide a convenient way to build your Flex projects using an industry-standard build management tool. If you are already using Ant projects to build Flex applications, you can use the Flex Ant tasks to replace your exec or java commands that invoke the mxmlc and compc compilers. If you are not yet using Ant to build your Flex applications, you can take advantage of these custom tasks to quickly and easily set up complex build processes for your Flex applications.
http://livedocs.adobe.com/flex/3/html/help.html?content=anttasks_1.html
Installation: http://livedocs.adobe.com/flex/3/html/help.html?content=anttasks_2.html#224215
Degrafa
Degrafa is a declarative graphics framework open source licensed under MIT.
http://www.degrafa.org
Download: http://www.degrafa.org/code/
as3corelib
The corelib project is an ActionScript 3 Library that contains a number of classes and utilities for working with ActionScript 3. These include classes for MD5 and SHA 1 hashing, Image encoders, and JSON serialization as well as general String, Number and Date APIs.
http://code.google.com/p/as3corelib/
Download: http://as3corelib.googlecode.com/files/as3corelib-.92.1.zip
Flex SDK coding conventions and best practices
Coding standards for writing open-source Flex framework components in ActionScript 3. Adhering to these standards makes the source code look consistent, well-organized, and professional.
http://opensource.adobe.com/wiki/display/flexsdk/Coding+Conventions
Flex Language Reference
The Flex 3.3 Language Reference provides syntax and usage information for every element in the ActionScript™ language. It documents all elements in Adobe® Flash® Player and Adobe® AIR™ that are officially supported by Adobe.
http://livedocs.adobe.com/flex/3/langref/
