It's F5, F6,
F8, F9, F10, F11 definitely. I never use the F-keys for anything except changing terminals, and I use only four text terminals, one graphical, and an error console. (Update: just realized that VNC uses F8.)
Pause is also mostly useless but a few games recognize it.
The SysRq magic is useful. Scroll Lock is sometimes useful to stop text (also it's the way to view the scrollback of the terminal on FreeBSD by default).
I definitely use Caps Lock. Its original purpose is not only useless but even harmful, as I press it accidentally too often. Thus, I've remapped it to write accented letters: it's easy because it's not too far to reach and I can hold it with my left hand while I type the accented letters with my right. It now works like this both on console and X.
I use Backspace, Escape, and # too.
Finally, I don't have a Windows key. I use a 101-button keyboard with no extra buttons on principle.
What's left? Ah, Mac key. I don't have one. Last time I had it was when I used a Commodore 64, although they called it the Commodore key there.
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