was built to unify the look of Mac OS X 10.4. The latest version of MacOS introduced a new interface design: the unified toolbar/menubar look. You can find examples of this look in the new Mail.app. This haxie will let you enable or disable the unified look of any Cocoa application (Safari, Adium, Colloquy are examples of applications that will profit from Unifier).
Since Unifier uses Unsanity's "Application Enhancer" preference pane, it'll leave your system free from any modification. All the customizations of your GUI are done on the fly and only in memory.