Swifty Compress and Decompress are separate commandline applications that can,
Compress and Decompress SWF files.

Swifty Compress and Decompress are separate commandline applications that can, compress and decompress SWF version 6 files. (Generated by Flash MX)

Notes

Version 1.0 of the utilities didn't work for SWF version 5 and below files because those versions cannot be compressed according to the official SWF specs. But the Flash player version 6 will play those 'illegal' compressed files fine. Version 1.1 does not check for SWF version and doesn't even display a warning about this situation. Version 1.0 download is still available and you can use the older version if you need to be careful about the SWF version.