Park Tool BT-2 4th Hand Tool
No other tool can mimic Park's 4th Hand Cable Stretcher. With one hand, this incredible tool pulls both gear and brake cables tight for maximized shifting and braking performance. Flip the thumb lock and your cables stay taut while your hands are free to tighten bolts and nuts. Features comfortable foam grips and chromed heat-treated tool steel construction.
The only time I use this tool is to tighten the ferrules on the housings after installing new cables and housings. I might use it to set up cantilever brakes ever once in a while, but that's about it. The thing about this tool is that you don't need it very often, but when you do, it's irreplaceable.
Works as advertised and worth the money to avoid the hassle of pulling cable by hand.