Flex888 summarized 10 major Flex frameworks implemented and can be used for Flex RIA developments.

They are:

Cairngorm (download)

PureMVC (download)

Model-Glue: Flex (download)

Foundry (download)

Guasax Flex Framework (download)

ARP (download)

Flest Framework (download)

There are others you can try them out, EasyMVC, Adobe FAST and Joeberkovitz’s framework. Moxie also mentioned a few other none-UI frameworks and tools to help out Flex developers, FlexUnit, FlexLib , AS3CoreLib and Granite DS.