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.
Most recent Flare version is 0.6.
Windows Explorer Shell Extension
Download flare06setup.exe. After installation right-click on any SWF file in Windows Explorer and choose Decompile from context menu. Flare will decompile somename.swf and store decomiled code in somename.flr in the same folder. somename.flr is a simple text file, you can open it with your favorite text editor. If Flare encounters problems during decompilation, it will display some warnings. If everything goes well, it will quit silently. That’s all, Flare has no other GUI. To unistall, execute Start>Programs>Flare>Uninstall.
Mac OS X Droplet
Get flare06.dmg. After mounting the disc image drop an SWF file onto the Flare icon in Finder. The decompiled ActionScript will be stored in SWF’s folder with FLR extension. Open it with your text editor. You can decompile multiple SWF files at once. The droplet is compiled on OS X 10.3. It should work on 10.2 and 10.4. There is no Flare for OS 9.
Command Line Versions
There is no installation procedure for command line versions. Just create a folder named flare somewhere and unpack the archive there. To uninstall, delete the folder and you’re done.
Or read more here.