ShapeShifter is a product that lets you change the overall appearance of your Mac using 'themes'. We're not talking about just desktop backgrounds and icons here, we're talking about everything - the look of windows, menus, apps, buttons, absolutely everything. You don't wear the same clothes everyday, your house doesn't look exactly like your neighbor's - why should the computing interface you use everyday be any different. You personalize your physical workspace to suit your tastes and whims, so why not your Mac?
NB: Shapeshifter is no longer supported! Read more: http://www.unsanity.org/archives/betas/mighty_mouse_with_some_theme_sauce.php