The latest release of the Mac4Lin project brings the Leopard look and feel to Linux. There are a number of steps you need to go through to get your GNOME desktop and GTK-based apps all looking consistent, but when you’re done, the makeover is quite convincing. The only giveaway is that menus remain attached to each window, instead of displaying at the top of the screen when a window is active.
Infra Red Dude wrote a thorough guide for installing Mac4Lin and tweaking all of your applications. He covers everything from upgrading your fonts and system sounds to changing the GRUB splash screen so that your Linux box boots with the Apple logo.