Moxie Zhang, over at InfoQ writes about Open Source Flex frameworks. [1]There are 7 listed frameworks, some of which are pure AS3 frameworks. This means you can use that particular framework outside of a Flex project.

One framework that got an honorable mention, is the EasyMVC framework by Simeon Bateman [2]. EasyMVC was designed to handle events inside a Flex application. It requires no boilerplate code and is very lightweight. Certainly something to consider when starting your next application. Enterprise applications require enterprise frameworks, like Cairngorm [3]. Less complicated applications naturally deserve a less complicated framework. Give a look to easyMVC.