EDIT:This is more about apps toolbars but it does change icons as wellCustomize Apps & Their Toolbars | View Screenshot
Love Apple's Mail app, but don't care for its toolbar icons? Using CandyBar's Application and App Extra modes, customizing applications and their associated icons is a snap. CandyBar scans your hard drive for installed apps and lets you drag replacements from the Finder or from Pixadex to quickly change them. Reverting back to your originals is quick, easy and safe too.