Utils/Misc/Large repos
Gaia Framework – Great framework for making websites, looks good!
as3corelib – encryption, picture formats and some networking classes..
as3 data structures – for game devs
as3FlexUnitLib – It’s like JUnit but for flex.
FlUint – Similar to Flex unit, but different.
OS Commodore 64 Emulator
Senocular’s Repo – Alot of nifty classes

Animation/Tweeners
TweenMax – best tweener class?
TweenLite – Lightweight tweener for use in flash banners and such.
gTween
Tweener – “Tweener is not awesome” says sekasi (I believe him.)

Cast 3D – 3D animation framework to use with 3D renderer like PV3D

File Types & Encryption
as3crypto – some more encryption classes..
Crypto – some more encryption…
fzip – modifying zips etc
ascollada – collada file type
as3syndicationlib – xml feeds (rss/atom)
GIF Player
GIF Encoder

Networking
loaders…
loader manager – great for preloading lots of stuff
Bulk Loader – Anyone know which loader is better?
AMF..
AMFPHP – amf protocol
ZendAMF – supported by zend and adobe
Database protocols..
assql* – mysql driver/utils for as3
assqliteutils* – sqlite utils
asql* – Another MySql library
Other Protocols…
XMPP – protocol for jabber/google talk etc
IRC – Internet Relay Chat protocol
VNC – Virtual Network Computing protocol & client
Flex FTP – File Transfer Protocol
POP3?
other..
http://puremvc.org/
http://opensource.adobe.com/wiki/dis…gorm/Cairngorm
http://mate.riaforge.org/

Website API’s
Youtube
Youtube data api – not sure how its different to the one above…
Youtube Chromeless api – A wrapper for the youtube video api
Flickr
Mappr – Flickr & Mapping
Amazon
eBay
Facebook
Odeo
Yahoo – lots of components and stuff also
Last-fm

UI – mouse/keyboard
Mouse Gestures
Senocular’s Key Class – Awesome lightweight class that recreates the as2 Key object.
Senocular’s Virtual Mouse

Sound
popforge – sandbox project.. it makes beats and rhythms… me thinks.
soundeditorlib – no idea..

3D graphics
Papervision 3D – Best 3D?
alternativa3d – Powerful 3d engine (free for non-commercial use).
Away 3D
Sandy 3D
FIVe3D

GUI/Components
flexlib – flex gui components
ASWing – Gui Component things.
Flare – charts and stuff..
SmileArea – inline pictures/smilies textarea component

2D Graphics/Particles
degrafa – Drawing tools and alike
htmlwrapper – a html rendering engine in flash… it looks very nice.
Flint Particles – some cool particles

Vector Drawing
µSprite – I’m not sure if this should be in here, its more like an app…

2D Physics
box2d as3 – Best 2D physics?
Motor2
ape
Foam
Glaze – aka exile remake inspired from n

3D Physics
wow engine – best 3d physics? (not for long)
jiglibflash – based on papervision & the c++ library called jiglib.

Tile/game engines
FFilmation
CDK – Collision Detection Kit (BitmapData collision checking)

Maths
The Canadian’s Extended Math – Does the same thing as default Math class plus more.
The Canadian’s Equation Parser – Give it a string equation and it’ll solve it  (requires extended maths class)

Other (Apps & haXe)
ASDoc – Generates Documentation for as3 projects (comes with the flex sdk/builder)
haXe – OOP language that can compile into swf, neko, php, js & c++
Policy File Server – haXe source code from ViktorHesselbom
haXe font manager class – haXe/flash class

If you got any to add to the list please post a link. 
or something better to replace something with..

some classes that would be nice to add to this list:
- advanced key binding class similar to that of valve games..
- mouse class?
- hybrid BitmapData/Bitmap class thats nicely written with a buffer etc.
- shapes like stars, diamonds… great for particle effects.
- text management? speech bubbles, bitmapData text etc

Note *:
I labeled the database classes with * because its more efficient and secure to use AMF protocol to connect to php then have php connect to the database.

This way the user will not have access to a database password if they decompile the swf/use a packet sniffer.
Also the AMF protocol is compressed binary, it uses alot less bandwidth then xml formatted results etc. (xml is still nice for usability reasons like rss feeds, SEO etc)

However you can still use the direct connection libs, just keep in mind the above and create a MySql account with SELECT only privileges.

Original link: http://www.kirupa.com/forum/showthread.php?t=315129