Besides basic theming changes to your desktop covering login windows, window look and feel, a modified app launcher dock, and more, Mac4Lin plugs into and plays nice with a lot of third-party apps. Thunderbird, Rhythmbox, Songbird, Pidgin, Firefox 3, and others are supported and receive theme integrations, while a customized Cairo or AWN dock are rolled into your system during the installation.
It goes without saying, but there are, obviously, areas where the Linux desktop can't replicate the Mac's. Still, if you like the Cupertino look for your system, Mac4Lin is an easy way to get there, without an afternoon-consuming step-by-step process.
Mac4Lin 1.0 is a free download for Linux systems. Hit the link below for a list of features, standing bugs, and screenshots.