I keep a copy of The Unarchiver around for those odd formats and password-protected archives, but I still leave most things to Archive Utility. I find The Unarchiver is still too buggy for me (things like http://code.google.com/p/theunarchiver/issues/detail?id=215 ).

Hmm why? I've never downloaded a zip it couldn't handle. And for creating, tar from the command line.

Then you've never downloaded a password-protected Zip file, a RAR file, a 7-Zip file, or an LZMA / XZ file. Nor have you needed to unpack a CAB, RPM, or XAR archive.

I've used 7zX for those rare cases, which is ugly but works. ...In case people don't want to replace the default Archive Utility.

that looks really cool; half the reason I wrote this was to find out what gems I am missing.

Wow, thanks for this. I've been using a Mac since 2000, and never came across this gem.