I have a custom keyboard I designed, and I configured it so that the tilde key turns the right-hand side into a numpad (ie the yuiohjkl etc keys). It's great, no need for extra unused buttons and very convenient to use (as the left hand is typically not used when typing on the num pad).
I used to have a Sony laptop like this, where NumLock turned the uiojkl keys into 456123, so you could use them (along with 789 above them) as a number pad. It was a clever design and I wish more laptops offered it.
