Mac OS X only: Spruce up your Mac's look with the OSX-flat icon set, which is split into black-and-white and colored icon sets. Coupled with previously mentioned LiteIcon, replacing the default icon set and theming your Mac has never been easier. Remember that if you use LiteIcon to replace your icon theme, you will need to relaunch Finder for the changes to take effect. OSX-flat is a free download for Mac only.
OSX-flat [Apple Downloads via Cool OSX Apps]