Mac4Lin brings Apple's Mac OS X UI to a POSIX Operating Systems (GNU/LInux, FreeBSD, openSolaris etc.), including details like app icons and even the Dock. Mac4Lin works on a variety of Linux flavors, including Ubuntu 9.04 and GNOME 2.26. A lot of third-party apps that use GTK for skinning, like Firefox and Songbird, look great in Mac4Lin. An unstall script is also included in the latest version, in case you decent you don't like the OS X look, and you'd rather go back to your default UI.
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