I don't know if I can come up with 5 really prominent ones, but here is my list:

(1) Papervision3D
(2) Degrafa
(3) Corelib
(4) PureMVC
(5) Mate

Well, I guess I could :)

1. Cairngorm: I'd have to say it is one of the top. Although most people are moving on to other architectures it really got that whole movement going.
2. Degrafa: it really opened up graphics way before fxg was even close to being around.
3. FlexUnit: bringing real unit testing to Flex.
4. FlexLib: it did a great job of demonstrating how to make a library of reusable components
5. PureMVC: although it isn't strictly Flex specific it had a major influence over architectures like RobotLegs and Mate that are so prevalent in Flex development now.

OpenScales : Flex & pure AS3 map viewer
Away3D : 3D engine
Spring ActionScript : IOC
Swiz : MVC, IOC, Event handling
Degrafa

(1) Swiz (can't live without it)
(2) Degrafa (still the best)
(3) AlivePDF
(4) FlexUint
(5) Flexlib

As I was typing this in, I see where you might be going with this. Either I just can't think straight right now, or there aren't that many out there.

I think Flex is too limiting, so I'm going to extend my list to ActionScript.

- Papervision3D
- Degrafa
- AS3Corelib
- Tweener
- AMFPHP

1.Birdeye
2.Flexlib
we are heavily rely on the birdeye project to visualize our data, and the flexlib components act as key components of our application.

We really thank all the developers of these opensource projects and helping us out accomplish the tasks which we couldnt imagine they could be done. A big THANKS to all of them again and also Adobe for building out such a Powerful and Flexible Platform... FLEX the GREAT :)

Swiz
PureMVC
Spice Factories Parsley Framework
as3lib
DistortionEffects
FlexSpy
AS3 API's (http://www.ericfeminella.com/blog/actionscript-3-apis/) used IMap and HashMap extensively in our application.
FlexLib

Madan Narra