Flare processes an SWF and extracts all scripts from it. The output is written to a single text file. Only ActionScript is extracted, no text or images. Flare is freeware. Windows, Mac OS X and Linux versions are available.

The main purpose of decompiler is to help you recover your own lost source code. However, there are other uses, like finding out how a component works, or trying to understand poorly documented interface. Depending on where you live, some of them may be forbidden by law. It's your responsibility to make sure you don't break the law using Flare.

If you develop Flash applications for living, you probably know that your code is not secure in SWF. It's not the existence of decompiler that makes your code insecure though, it's design of SWF format. Although no ActionScipt is stored there, most of it can be recovered from bytecodes.