I've once seen it on TV: in industrial wire production they use a set of castors that bends the wire back and forth, up and down – always a bit less, until its straight. Is there something similar for home use?
I don't know about the machine, but depending on the size of the wire and the length, you can straighten it by holding one end in a vise and then holding the other end in a pliers and giving it a quick jerk. It takes a little practice but it works quite well.
However I would hesitate to use this technique on wire you plan to use for household wiring. If the wire gets stretched too much the diameter of the conductor could be reduced, and likewise the ability to carry the rated current.