Mac OS X: MenuPop is a free app that makes your menubar accessible as a contextual menu, letting you access menu contents easily via the keyboard.
The app is very simple: you just press a key combination you set yourself within MenuPop and it'll pop up the contents of the current menubar. Although it shows up at mouse point, it's not necessarily most effective to navigate with your mouse. Since you can only activate it with a key command, it makes it easy to just keep going through each menu with your keyboard. If you do more with your keyboard than your mouse, MenuPop is incredibly useful. While MenuPop gets the job done just fine, if you want even more options and are willing to pay for them, Binary Bakery Software offers up Menu Everywhere for $US15.
MenuPop works with Mac OS X 10.6 and up, and you can download it for free right now.