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PrtSc/Sys Req: I do very very rarely use this key, much more on *nix than Winboxen because it puts an image in a folder rather than that clipboard stuff.
Scroll Lock: I'm going to have to go with this. I just don't tail fast moving logs often enough to get excited about it.
Caps Lock (always leave it off): In fact IO quite often have to pull this key off (why it is so out sized on some keyboards I have no idea but my typing isn't all that aND THIS EFFECT normally follows at some point)
Pause/Break: I still use this, mainly when trying to get out of windowsNT batch files (written for places that I can't deploy activeperl or anything else for that mater)
The 'Windows' key (because I run Linux): My Windows key works in Linux! There is a fix for it <google> and I've got a small penguin sticker.
The 'Mac' key (same reason): I have no such key <shrug>
# (comments are for sissies): Use POD or something - what do you do when the semi-colon key breaks?
F12 (hard enough to remember what F1 does): Someone has already said you need this key to access a Dell BIOS; I don't have a Dell but everyone else on the sodding planet seems to.
Esc (I prefer to face my demons head on): I use vim, you can't have my Esc key :qw.
Backspace (I never make mistakes): How would I fix the mistakes I make with the cAPSLOCK key without this one?
BUT what is Alt Gr for?