Adobe Flex is a great platform to work with for creating rich engaging applications of all sizes for the web and now the desktop with AIR. Now we are at Flex version 3 and the next version Flex 4, which is the SDK, and the editor now called Flash Builder are just around the corner. There are a number of resources, guides and articles that I use regularly when building Flex or AIR applications. This is a growing resource so bookmark the page and feel free to suggest anything that I may have missed. Here we go then:

Explorers

• Flex 3 style explorer (Adobe)
• Tour de Flex - Flex 3 component explorer also available as an AIR application (Adobe)
• Regular Expression explorer
• Effects explorer (James Ward)
• Designer effects for Flex components (Tink)

Guides and cheat sheets

• Flex 3 Developers Guide (Adobe)
• Flex 3 Language Reference
• Flex Builder cheat sheet (OS X)
• Debug a remote application in Flex Builder

Coding standards, conventions and best practice

• The guidelines used by the Adobe team on the Flex SDK
• Flex best practices part 1 (Adobe DevNet)
• Flex best practices part 2 (Adobe DevNet)
• Top 10 Flex mistakes

Flex performance

• AS3 and Flex optimisation techniques (Inside RIA)
• Improve client performance (Adobe DevNet)
• Creating smaller SWF's (3 part)

AIR performance

• Idle CPU usage in AIR (Grant Skinner)
• Writing efficient AIR applications (AIR core team developer)
• AIR performance tips (Adobe AIR blog)

Styling & skinning Flex

• Flex 3 CSS properties list
• Degrafa - declarative graphics framework for AS3 and Flex
• Axiis - data visualisation in open source

External Libraries

• Flexlib - open source components (Google code)
• AS3CoreLib - open source core library (Google code)

Further help

• Flexdeveloper.eu forum
• Adobe community forum for Flex
• ActionScript.org
• Adobe Flex Cookbook

This is just a small list that you can use everyday. It's not definitive and does not try to be.