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Pop Up Main Menu
Overview
At least for me, one of the best and most ergonomical NeXTStep features used to be the possibility to pop up the main menu anytime, anywhere, without a need to run the mouse over the whole screen (actually, I've even learnt to hide the main menu altogether–whenever I needed it, I had popped it out).
For a long, long time I have tried to mimic this behaviour with Mac OS X. It was not easy by far, since Mac OS X does not allow you to catch a mouse event which happens to be triggered over a screen space which does not belong to the application; finally though, I have been lucky to find a trick to do it.
User Interface
The user interface is quite straighforward: just press the ctrl-⌥ modifiers at the same time – the mouse cursor changes to a special menu cursor, which looks like this:

Whilst the cursor is shown–in other words, whilst keeping the ctrl-⌥ modifiers pressed–click or press the mouse. The main menu pops out at the current mouse position, and allows you to select items the standard way: by dragging if you popped up the menu by a press, by pointing and clicking if you just clicked to pop it out:

As soon as the menu pops out, you can release the modifiers.
If you happen to have a two-button mouse, you can use its right button to pop out just the Services menu (which is normally not quite easy to reach, being hidden inconveniently deep in the menu hieararchy).
Customization
At this moment, there is no customization available through GUI. Power users can change the modifiers using the defaults system–see please the Power User Help page for details.