irrFuscator is an ActionScript obfuscator. It protects .SWF files as well as .AS and .MXML files, no matter if you use Flash or flex. irrFuscator is available for the command line but also comes with a convenient GUI frontend. It is able to encrypt strings, obfuscate method, variable and class names, strip whitespace, does nondeterministic obfuscation and automaticly detects references to external flex code.

The ActionScript 3 Byte Code (abc) in .swf files still contains a lot of symbols revealing how your program works and what it does, so it is very easy to reverse engineer or steal your code. irrFuscator can be used in two modes:
SWF Obfusction (Binary): protects an existing .swf file.
Flex Obfuscation (Source): reads in a directory of .as and .mxml files, processes them and puts them out into a target directory. Simply recompile the directory with the obfuscated files and your .swf files are a lot safer.