Mac OS X Theme 1.3
I'm about to convert to Mac myself but if I wasn't, I would definitely use Mac OSX Tiger Theme 1.3 to at least feel like I'd converted.
The skins feature the classic Aqua and Graphite schemes and the newly introduced Platinum scheme which you can find in the System Preferences and Mail programs in Mac OS X 10.4. They've also included Brushed with over 20 substyles added for good measure. The Apple menu is unfortunately, not an exact copy of the original but it's pretty close. It's basically a stripped down Start Menu that has been made to look like the Apple menu in Mac OS X. In addition, there are also left and right handed titlebar buttons for those that want their PC to look like a Mac but at the same time keep their right handed Windows buttons. The look and feel is remarkably Apple although Windows keeps creeping through especially with it's annoying sounds sometimes to spoil the illusion. If you have any widgets installed, you may also find that they don't show with this installed.
A great skin for those that either can't afford a Mac or those that simply prefer the elegant Mac look and style.