Here are Flex reference materials — blogs, component libs, videos, etc. Most are excellent and some (like the Flex Examples blog) are indispensable. I also highly recommend Adobe TV.

Some useful Flex sites:

• One of my favorite Flex sites is Peter deHaan's Flex Examples. Clearly written, live examples to test drive, source code included. He has another blog, Confessions of a Flex QA, which provides general Flex/Flash news, an aggragation of good posts from other Flex sites/blogs, and summaries of his own Flex Examples.
• The Adobe Developer Connection site Devnet has lots of info, including many white papers.
• Flex.org used to be at the bottom of this list, but it's been revamped and it's now an excellent resource for articles, tutorials, and more.
• The Flex Doc team blog has a wealth of information and examples.
• Alex Harui's Flex Closet is another great resource, he's knowledgable on a lot of topics but especially check him out if you have q's on working with ItemRenderers.
• Mihai Corlan's site has a lot of good content for both Flex and AIR; especially good if you are using PHP.
• Ely Greenfield's site quietlyscheming is especially good if you're interested in graphics and effects, not only has great code but he even has how-I-did-it demos.
• If you want to integrate Flex and Java you should check out James Ward's site. He also has a great comparative analysis of Flex data access methods here.
• Doug McCune's blog Always practice safe Flex is definitely meta-geeky (using a decompiler to decompile itself?!)
• Check out The Flash Enabled blog which also has Flex stuff, including an interesting page Flash & Flex Free Components and Source Files
• Also useful is Actionscript.org, a good place to post questions on AS problems you're having.

Tour de Flex

Tour de Flex deserves a category to itself. This AIR app can be downloaded from Adobe and provides tons of interactive examples with source code, everything from demonstrations of basic component functionality to advanced data access methods (including some that employ 3rd party products like WebORB). Highly recommended, especially for newbies.

Update: For mobile app developers there is now a Tour de Mobile Flex — download it from the Android app market.

Online Tools and Component libraries

• To see Flex 3 components in action go to The Flex 3 Component Explorer (some of the same functionality you'll find in Tour de Flex).
• The Flex Style Explorer lets you play with CSS settings for mx components. When you get just the right shadow and corner radius on that gradient pink button this utility can generate the CSS for your selections, all ready for pasting into your project's CSS file.
• Some libraries of Flex components: Flexlib, Flexbox, Flex.org components lib, also Devnet.
• Some libraries of Flex skins and templates: Fleksray and Scalenine.

Interesting RIA/Flex Podcasts and Videocasts

You can find lots of resources on iTunes and YouTube, but here are some specific goodies I want to highlight:

• Adobe TV has incredible videos, including many presentations from the Adobe Max conferences (want to understand the UIC lifecycle and creation of custom components in Flex? Check out "Creating New Components in Flex 3" by Deepa Subramaniam)
• A good place for newbie podcasts is The Flex Show blog. This also has more advanced tips, developer interviews and Flex conference presentations.
• Adobe Developer Connection videos are short, focused videos by Adobe staff, available on both Adobe TV and iTunes.
• Adobe provides a nice (and free) set of accelerated Flex training videos on its devnet page Flex in a Week
• Among other Flex goodies, Ted Patrick's site has numerous videos
• Lee Brimelow, an Adobe Flex evangelist, maintains gotoandlearn, a site with videos on a very wide variety of Flex/Flash/AIR/AS topics.