I used to have a keyboard which completely lacked a Scroll Lock key. Aside from that, the only useless key I have is Pause/Break, since I've never had the need to press it. Same goes for the number row, aside from the punctuation marks they provide; numpads are just easier for me to use.
Actually, saying that, I think the one key I can not find an appropriate use for is the key to the far left of the number row: `/¬ (prime/backtick, not symbol). I've only ever used it for making this emoticon.
¬_¬
Probably the num lock, never used it, never will, I mean I have no idea what it does anyway.
Scroll Lock, I never use this key.
i have also never used the scroll lock key. i guess this is really the most useless key of all.