I summarized some open-source projects about Flex.

Adobe APIs

1 corelib:consists of several basic utilities for MD5 hashing, JSON serialization, advanced string and date parsing, and more.
2 FlexUnit:FlexUnit is a unit testing framework for Flex and ActionScript 3.0 applications. It mimics the functionality of JUnit, a Java unit testing framework, and comes with a graphical test runner.
3 Flickr:The Flickr library is an ActionScript 3.0 API for the online photo sharing application, Flickr. It provides access to the entire Flickr API.
4 Mappr:Mappr is a service and application that combines images from Flickr with geolocational information. The Mappr ActionScript 3.0 API gives you access to Mappr’s geo-tagged image data.
5 RSS and Atom libraries:Use the RSS and Atom libraries to parse Atom and all versions of RSS easily. These libraries hide the differences between the formats so you can parse any type of feed without having to know what kind of feed it is.
6 Odeo:The Odeo API provides an ActionScript 3.0 interface for searching and retrieving podcasts from Odeo.
7 YouTube:The YouTube API provides an ActionScript 3.0 interface to search videos from YouTube.

as3awss3lib
This is an AS3 library for accessing Amazon’s S3 service. It only works in Apollo because of restrictions in the browser player.

as3soundeditorlib
Application displays a spectrum of the mp3 file, displays cue points from a file, and allows navigation and playback of mp3 and navigation between cue points. MIT-license.

as3ds
AS3 Data Structures For Game Developers’ is a library containing data structures optimized for game development with Adobe Flash and Actionscript 3

mecheye-as3-libraries
consists of a set of ActionScript 3 libraries used primarily for Flash game development. These libraries are known to work with the Flex 2 SDK.

APE
a free AS3 open source 2D physics engine for use in Flash and Flex, released under the MIT License.

As3Crypto
a cryptography library written in Actionscript 3 that provides several common algorithms. This version also introduces a TLS engine (TLS is commonly known as SSL.)

FZip(or here)
an Actionscript 3 class library to load, modify and create standard ZIP archives. FZip parses ZIP archives progressively, allowing access to contained files while the archive is loading.

ASCOLLADA
A actionscript library for parsing Collada files,released under the MIT License

Away3d
Away3D is a realtime 3d engine for flash in actionscript 3, originally derived from Papervision3D.

Papervision3D
a realtime 3D engine for Flash

uicomponents-as3
a lightweight AS3 UI component library

Tweener
a Class used to create tweenings and other transitions via ActionScript code for projects built on the Flash platform

asinmotion
An animation library for actionscript 3. Contains a basic tweening class as well as the ability to sequence and execute tweens in parallel

flest
an ActionScript3 / Flex application framework for building enterprise level RIAs. It uses such design pattern as Controller, Factory, Command, etc. High efficiency, simplicity and practicality were set as its mandatory design features.

facebook-as3
Implementation of Facebook’s REST API in Actionscript 3.

Salesforce Flex Toolkit
allows Flex programmers to build S-controls — components within the Apex interface where custom code can be executed — without dropping into JavaScript

Twitter AS3 API
dependency is the Adobe core library

XIFF
XMPP client library

Midas3

a pure actionscript3 library that provides seriliazition/unseriliazition APIs and some MIDI node event APIs.