If you use a recent Chromium build or Google Chrome dev channel, you can enable this feature by adding a command-line flag to the desktop shortcut: --new-wrench-menu.
To make the menu more compact, Google uses a single menu item for cut, copy, paste and another menu item that combines zoom options with full-screen.
Opera already uses a unified menu that replaces the menu bar, while Firefox 4 will include a single menu button. The unified menu takes up less space, it's less complex and it reduces clutter.
"The general purpose of the menubar is to contain all of the things that you want your program to do but you can't cram into the main UI. So the menubar generally ends up with a lot of stuff that isn't used very often, if at all, and yet is reproduced on every window and takes up a significant amount of real estate. It also has the tendency to become a dumping ground for new or hardly used features. Starting with Vista, and continuing with Windows 7, the menubar has been systematically removed from Windows applications built by Microsoft and other vendors. It has been replaced with alternatives like the Windows Explorer contextual strip or the Ribbon found in Office 2007," explains Mozilla's wiki.