mapping ESC versus META

[Christopher C. Stacy]

I run several different versions of Emacs, 
and sometimes there is an actual key to be META.
Is there a way I can bind a command to that key
chord combination, but bind a different command
to the ESC sequence?    In other words, when I
hold down the META shift key and press letter "Z",
I would get one command, but when I type "ESC Z" 
I would get a different command.

On the Macintosh, the META key might be either
the Command or the Option key.    As it happens,
I always have it set to be the Command key.
I want this to work under X11 on the Mac, and Linux.
It would be best if it also worked for MS Windows.
The situation where I may not a META key is when
running Emacs under the various Terminal programs
on those various platforms.