If incremental customization isn't what you're after, ShapeShifter ($20) - the neutron bomb of desktop customization - lets you make major changes. This app from Unsanity adds Windows-like theme support to Mac OS X. New themes alter the entire look and feel of your Mac, from window buttons to folder icons, so be sure you're committed to major renovations before proceeding.

Once you've installed ShapeShifter, get started by choosing its pane in System Preferences. Click the Download tab to browse available themes. Download a few that appeal to you and scroll through them using the navigation buttons in the lower-right. Click the Apply tab when you land on the one you want. Select a new theme, icon set, and desktop image from the drop-down menus on the left, and click Apply Changes. The Finder and Dock will restart to reflect the new look. You'll need to restart any open applications yourself. To return to familiar old Aqua, just click Restore Defaults in the ShapeShifter pane in System Preferences. The newest version, ShapeShifter 2.4, is a Universal app.

Themes like Milk, pictured here, change the look of file menus, application windows, and icons.