Zlash is an ActionScript decompiler / compiler for Flash .swf files up to version 7. With this software, you can get the source code of any ActionScript, save it in a text file, modify it (using the included editor or your own favourite source code editor), recompile it and produce a new modified .swf file even if you don't have anymore the original .fla project file.
) "Zlash - Click to enlarge screenshot")