Rarify is a very simple interface to the rar binary and RAR is a popular compression format in the PC-side-of-the-life. The RAR format won't support OSX-specific flags (ie, resource fork). For advanced options, use the rar binary in CLI mode itself, or look for other interfaces. You can use the Preferences window to adjust some settings (password if needed, segments if needed, etc.) and then drop onto Rarify's icon your file(s)/folder(s) in order to compress them. Rarify will run under OS X 10.4 and 10.5, both Intel and PPC architectures.
