DejaMenu will display the current application's main menu as a context menu when a key combination is pressed.
This is particularly useful for people using multiple monitors. Since the Menubar only appears on a single monitor, it is awkward to use applications on a different monitor. DejaMenu solves this - just press the configured key combo, and a copy of the Menubar is right there for your to use.
If your mouse can simulate key-presses, DejaMenu becomes even more useful. For instance, on my 4-button trackball, I have the fourth button set to bring up DejaMenu by simulating shift-cmd-m when clicked. (USB Overdrive and Kensington Mouseworks both allow mapping of keys to button presses).
In many ways, this was inspired by the NeXTStep convention of having the main menu appear on a right-clic