So you're playing around with a brush or pencil and want to draw some straight lines. Good luck doing it by hand. Ordinarily you'd bring up a grid and turn on snapping, but if you're not that committed, try the following approach. With the brush or pencil selected, hold down SHIFT and left-click once at the point where your line should start. Then left-click again at another point. Photoshop will draw a straight line from A to B. Neat! And there's no reason to stop there. You can keep going through to C, D, etc.