Sometimes its necessary to code in AS2 to target Flash Player 8, so we're forced to write within the Flash IDE.
Any refactoring tools / plugins available for Actionscript?... Within or without the Flash IDE, that's no problem, as long as it builds for Flash Player 8, AS2.
FlashDevelop looks good, and you can set it to build using the Flash IDE.. (It simulates ALT+TAB and CTRL+ENTER!)
And as to its AS2 support, you can simply #include the AS file that you write within FlashDevelop, into your main FLA.. so your code gets compiled and published.
The open source MTASC ActionScript 2 compiler does not require the Flash authoring tool. Jeremy Rudd mentioned FlashDevelop, a cool free IDE for SWF development. I used it extensively when I still coded in AS2, and I definitely recommend it. It supports compiling with either the Flash authoring tool or MTASC, which is nice. Recent versions support AS3 development with the free Flex SDK too. It's Windows-only, though, so it may not be useful for all developers.
I don't think FlashDevelop has any sort of refactoring support yet, though. Since most developers are focusing on AS3, it's likely that advanced IDE features, like refactoring, probably won't be implemented for AS2 in most editors.