There has been a huge uptick in activity in the Flex Component space. It seems that many developers are making components and extending Flex in unique ways. Four projects to highlight are FlexLib, FlexBox, FlexComponents List, and Flex Exchange.
FlexLib
FlexLib is a source code repository for Flex components under an MIT license. The repository contains there open source components: ConvertibleTreeList, Draggable Slider, PromptingTextInput, Scrollable Menu Controls, SuperTabNavigator, Alternative Scrolling Canvases, Horizontal Accordion
FlexBox
FlexBox is a directory of 101 Flex components in a Flex RIA. It is a great source for locating various Flex components scattered all over the Internet.
FlexComponents Discussion List
If you are making components or extending Flex this is the mother of all lists. Backed by some of the framework engineers and packed with great devs, FlexComponents is a great resource.
Flex Exchange
Flex Exchange at Adobe.com is a directory of Flex components submitted by developers.
Cheers,
Ted :)

DIGG IT!
Hi Ted,
Just thought I'd point out to your readers that although FlexBox right now has 101 Components, the idea is to get as many as we can find in there. The current set of components that you see in FlexBox came from the bookmarks that I collected in my 8-9 months spent with Flex. Surely there are others who have been collecting similar bookmarks and may have pointers to components that are not yet in FlexBox ... If you do know of any such components leave me a link/other details on my blog entry introducing FlexBox or contact me using these details.
Note: FlexBox right now is FlexBox 0.01 and hence we do not have an inbuilt form for adding components, I will try and add that soon.
Ted,
At some point in time, sooner than one might think, there will be a need for a central component repository in which developers can both publish, query and retrieve versioned components (.as, .mxml, .swc, .swf). For comparison, both the Java and Ruby community have well-known and cleaver solutions to this. Look at Maven (Java) or RubyGems and read about their features. Both implementations are completely open-source driven. One particular common aspect is the notion of component dependency stored in meta-data. This way a component developer can define exactly what other versioned components are required for the user of this version. Both Maven and RubyGems provide tools to access those components and store them in a local repository for use in projects.