Ubuntu and distro derivatives like Linux Mint can turn their GNOME desktop into Mac OS-style aqua interface with Mac4Lin. Other programs like Skype and Google Chrome need some workarounds though.
For Skype, click Options -> General -> Choose style: GTK+
For Google Chrome,
1. click Preferences -> Personal Stuff -> Appearance (Use system title bar and borders)
2. download the Mac4Lin Google Chrome extension and install. (Note: The scrollbar needs a fix)
1. install the MacOSX add-on at https://addons.mozilla.org/en-US/firefox/addon/12782/
Note: Ubuntu users can download Mac4Lin-based Macbuntu instead.