The parade of Flash API is presented here.
Papervision 3DPapervision is most popular open source realtime 3D engine for Flash.
Downloadhttp://code.google.com/p/papervision3d/
Sandy3DSandy3D is another open source 3D API for flash AS2,AS3.
Downloadhttp://flashsandy.org/download
Away3DAway3D is next generation realtime 3D engine for Flash. Away3D is very advanced and uses GPU rendering,
Downloadhttp://away3d.com/downloads
Facebook APIFacebook API for Flash, Flex and AIR.
Downloadhttp://code.google.com/p/facebook-actionscript-api/
More Resourcehttp://www.adobe.com/devnet/facebook.html
Tweener APITweener class is useful for creating code based dynamic animations and transitions.
Downloadhttp://code.google.com/p/tweener/
Documentation
http://hosted.zeh.com.br/tweener/docs/en-us/
Greensock Tweening PlatformAmazing code based tweening platform for all. This is having various options like TweenNano, TweenLite,
Document, Downloads and Resourceshttp://www.greensock.com/v11/
Box2D APIBox2D is very popular physics engine for flash game developers.
Downloadhttp://sourceforge.net/projects/box2dflash/
Documentation
http://www.box2dflash.org/docs/
Stardust Particle EngineStardust is wonderful particle engine for flash. It is useful for creating 2D and 3D particle effects. Some of my personal favorites are waypoints, exploding image and star fireworks.
Download and Documentationhttp://code.google.com/p/stardust-particle-engine/
amMAPAmMap is a tool that lets you have good-looking and functional maps on your website. The tool use AdobeFlash technology for a compelling online experience. You can display maps in any HTML page (web page), or inside another Flash movie. You can also use the maps in PowerPoint presentations.
Downloadhttp://www.ammap.com/download
Documentationhttp://www.ammap.com/docs/
Giggya APIGiggya is a perfect tool for sharing your flash content on social network like Facebook, Twitter, LinkedIn, Yahoo!,
Google Buzz, Windows Live, opened etc.
Documentationhttp://developers.gigya.com/
WiiFlash APIEver think of playing flash games with Wiimote? If yes, than this is the API for that. This consist of two parts WiiFlash Server (for getting input from Wiimote and send it to Flash and vice versa) and WiiFlash ActionScript API (for using Wiimote events in Flash)
Downloadhttp://code.google.com/p/wiiflash/
FLARToolKitFLARToolKit is a AS3 version of ARToolKit, which is a software library for building Augmented Reality (AR) applications. FLARToolKit recognize the marker from input image and calculate its orientation and position in 3D world.
Download, Resource, Exampleshttp://www.libspark.org/wiki/saqoosha/FLARToolKit/en
Player.IOPlayer.IO is a support tool service for multiplayer game, database service, connectivity for Facebook, solution for in-game payment. It supports AS3, .NET and Unity3D. It’s one stop solution for those who are looking for affordable multiplayer game serving.
Downloadhttp://playerio.com/download/
Documentation
http://playerio.com/documentation/
Come2PlayCome2Play platform is for single player as well as multiplayer. It’s having features like leader board, viral messaging, in-game money, banner ads, and lobby for multiplayer games.
Download and Resourceshttp://www.come2play.com/developer.asp
Source: Flash And Flex Developers' Magazine, May 2011.