Please mind the mess while I construct a properly listing of all existingAS3 Libraries/APIs:

corelib - The corelib project consists of several basic utilities for MD5 hashing, JSON serialization, advanced string and date parsing, and more.
http://code.google.com/p/as3corelib/

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.
http://code.google.com/p/as3flexunitlib/

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.
http://code.google.com/p/as3flickrlib/

Mappr - 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.
http://code.google.com/p/as3mapprlib/

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.
http://code.google.com/p/as3syndicationlib/

Odeo - The Odeo API provides an ActionScript 3.0 interface for searching and retrieving podcasts from Odeo.
http://code.google.com/p/as3odeolib/

S3 Amazon ActionScript Classes - Connect to Amazon’s AS3 service from Apollo
http://code.google.com/p/as3awss3lib/

YouTube - The YouTube API provides an ActionScript 3.0 interface to search videos from YouTube.
http://code.google.com/p/as3youtubelib/

Adobe APIs
corelib, mappr, flickr, youtube and more.
http://labs.adobe.com/wiki/index.php/ActionScript_3:resources:apis:libraries

APE (Actionscript Physics Engine)
http://www.cove.org/ape/

as3awss3lib
ActionScript 3.0 library for interacting with Amazon S3
http://code.google.com/p/as3awss3lib/

as3soundeditorlib
Actionscript 3.0 library for sound editing
http://code.google.com/p/as3soundeditorlib/

as3ds
AS3 Data Structures For Game Developers
http://code.google.com/p/as3ds/

ASCOLLADA
http://code.google.com/p/ascollada/

As3Crypto
ActionScript 3 Cryptography Library
http://crypto.hurlant.com/

asinmotion
Animation Library for AS3
http://code.google.com/p/asinmotion/

Away3d
http://code.google.com/p/away3d/

ebay API
http://code.google.com/p/as3ebaylib/

facebook-as3
AS3 API to access Facebook’s Platform API
http://code.google.com/p/facebook-as3/

flest
Flest Framework for Adobe Flex and ActionScript3 Applications
http://code.google.com/p/flest/

FZip
Actionscript 3 class library to load standard ZIP archives and extract/decompress contained files.
http://codeazur.com.br/lab/fzip/

lastfm-as3
Actionscript 3.0 library to access the Last.fm web services
http://code.google.com/p/lastfm-as3/

MapQuest
http://company.mapquest.com/mqbs/4a.html

mecheye-as3-libraries
A set of ActionScript 3 Libraries, primarily for Flash game development.
http://code.google.com/p/mecheye-as3-libraries/

Papervision3D
http://code.google.com/p/papervision3d/

Salesforce Flex Toolkit
http://wiki.apexdevnet.com/index.php/Flex_Toolkit

Tweener
Full featured animation library
http://code.google.com/p/tweener/

Twitter AS3 API
http://twitter.com/blog/2006/10/twitter-api-for-flash-developers.html

uicomponents-as3
Lightweight AS3 UI component library
http://code.google.com/p/uicomponents-as3/

XIFF
XMPP client library
http://svn.igniterealtime.org/svn/repos/xiff/branches/xiff_as3_flexlib_beta1/

Yahoo AS3 APIs
http://developer.yahoo.com/flash/

From: http://seantheflashguy.com/blog/2007/08/13/list-of-22-actionscript-30-apis/

Animation System v2.0
http://www.boostworthy.com/blog/?p=170

PopForge
http://code.google.com/p/popforge/

AsWing - GUI Framework
http://www.aswing.org/

Modest Maps
http://www.modestmaps.com

WOW 3D Engine
http://seraf.mediabox.fr/wow-engine/

Tween Lite
http://blog.greensock.com/tweenliteas3

Damn kid, that’s a lot of links.

Yahoo! just updated their Flash offerings:
http://developer.yahoo.com/flash/

Facebook AS3 API
http://code.google.com/p/as3facebooklib/

Open-Source Commodore 64 Emulator
https://mirror1.cvsdude.com/trac/osflash/fc64/

3D graphic engine

Papervision3D
http://papervision3d.googlecode.com/svn/trunk/

Sandy
http://sandy.googlecode.com/svn/trunk/sandy/as3/branches/3.0.1/

Away3D

3D physic engine

WOW
http://seraf.mediabox.fr/wow-engine/

2D physic engine

APE
http://ape.googlecode.com/svn/

FOAM
http://code.google.com/p/foam-as3/

BOX2DFLASHAS3
http://sourceforge.net/projects/box2dflash

Adobe AIR Libraries/APIs (http://weblogs.macromedia.com/cantrell/archives/2008/01/update_on_my_ai.cfm)

Cast3D - 3D animation framework library for Adobe Flash in ActionScript3.
http://www.cast3d.org/
AS3: DistortionTweener
http://evolve.reintroducing.com/2007/11/18/as3/as3-distortiontweener/
Quartz2D drawing API (90%) implementation in as3
http://code.google.com/p/ascanvas/

Dopplr combined with Modest Maps -> idea: world map surface with interactive travelling connections

Dopplr - Intelligent travel social network api
http://code.google.com/p/dopplr-as3/

Modest Maps - (previously posted by c.moore)
http://www.modestmaps.com/

Freaky 3D isometric game engine :

http://www.ffilmation.org/website/downloads/

http://code.google.com/p/skovalyov/

http://code.google.com/p/beni-as3-library

http://code.google.com/p/flexforum

http://code.google.com/p/mediacorelib/

http://code.google.com/p/xpath-as3/ ( Love that one)

http://code.google.com/p/flexstuff/

http://code.google.com/p/danielhai/

http://code.google.com/p/nobunaga

http://code.google.com/p/xpresso-flex/

http://code.google.com/p/blindside

http://code.google.com/p/flexundoredo/

http://code.google.com/p/cani-mindmap

http://visgraph.cs.ust.hk/MoXi/

http://code.google.com/p/lafabrick/

http://code.google.com/p/peoples/

http://code.google.com/p/as3libs/

http://code.google.com/p/flex-richtexteditor-html-utils/

http://code.google.com/p/picasa-flex-api/

http://code.google.com/p/nascomaslib

http://code.google.com/p/fxadata

http://code.google.com/p/meebone/

http://code.google.com/p/activepoison

http://code.google.com/p/as3preferenceslib

http://code.google.com/p/coderia

http://code.google.com/p/geronimo-air

http://code.google.com/p/fx-windowbounds/

http://code.google.com/p/masapi/

http://code.google.com/p/asqlib/

2.5D AS3 Isometric Engines

• Ffilmation

3D Engines

• Papervision 3D
• Sandy 3D
• Away 3D
• Wire Engine 3D
• Electric 3D
• Alternativa Platform

3D Game Engines

• PaperWorld 3D
• SWFZ

3D Vector Engine

• Five 3D

3D Physics Engine

• WOW Engine

3D Flash Components

• FreeSpin 3D
• Papervision3D CS3 Component

3D Animation Framework

• Cast 3D

3D Modelers

• G-Nero

http://code.google.com/u/christian.cantrell/

i add my own app, google maps multi-touch: http://code.google.com/p/mapstouch/

not to forget, the great visualization library flare: http://flare.prefuse.org/

check out the demos: http://flare.prefuse.org/demo/