Although I voted for "Caps Lock (always leave it off)", what actually happens is -- I get so tired of accidentally hitting it once in a while, that I finally disable it on all computers I use (Linux and Windows both). If I ever need to do all caps (and I can't remember the last time I did!), I just hold down the shift key with whichever little finger I'm not using.
in reply to The most useless key on my keyboard is:
So for me it's actually a "worst than useless" key, it's the only one I have to expend an effort to disable!