Windows: If you like the look and feel of Mac OS X but prefer to stick with Windows (whether it be for gaming, or because you just don't like Apple), freeware Snow Transformation Pack will make Windows looks like OS X.
Click on the image for a closer look.
Snow Transformation Pack will change the appearance of your whole system, including the login screen, icons, wallpapers, sounds, dock, dialog boxes and other UI elements. You'll need to disable User Account Control to install it, so you may have to run the installer twice — once to disable UAC, and then again (after restarting) to install the package. On installing, you'll get a few choices as to whether you want to install a few extra background apps that add extra graphics, which is nice if your computer isn't necessarily a powerhouse (so you can turn them off to keep performance up).
It looks surprisingly like OS X, with a few exceptions in usability. There are a few manual tweaks you can make to make even look more Mac-like (such right clicking on the Taskbar, hitting Properties and checking "Use Small Icons" in the Taskbar tab), but overall it looks pretty good. If you're tired of the usual Windows GUI and want to get a Mac-like experience for a while, it's a neat and easy tool to help you do so.
Snow Transformation Pack is a free download, Windows Vista/7 only.