+1 for Swiz http://swizframework.org/
It has built in support for Spring like dependency injection, event mediation, and very simple controller layers that pass on your calls to the service objects.
It is relatively new, but I found it to be very stable, and the dev team is very responsive - http://groups.google.com/group/swiz-framework
(requires google login).
The best thing about it is that it does not try to too much itself, but makes it easy for you to write good Flex code.
Our team had started off using Cairngorm, and I was looking for better alternatives. I had a brief look at Mate and PureMVC before I delved deeper into Swiz, and liked it immediately. From Java land, it makes me think of Stripes Framework, whereas PureMVC or Mate will be like Wicket or Tapestry