Well, the best open source editor for ActionScript projects seems to be
FlashDevelop , which is reviewed here...
http://www.flashmagazine.com/1243.htm
These tools allow you to compile a SWF using ActionScript and no FLA. You have to dynamically attach all of your Flash Components programmatically via class-based programming, rather than via a "pretty" graphical interface.
If you are familiar with Java programming techniques, then these tools will be familiar. The compilers used are often more concise than the Flash IDE, and therefore lead to somewhat better coding.
There are others (described in the above review), but I've found most of them difficult to use. See also
http://osflash.org/projects for other open-source initiatives centered around Flash development.
But, as the other posters mentioned, there doesn't seem to be anything else that will give you the ability to edit an FLA graphically by dragging-and-dropping components onto a stage.
Hope that helps!