Q. Is it possible to draw a straight line with the brush tool in photoshop?
A. Yes, hold shift whist you click and drag the paint brush across the canvas. If you choose a 1 px brush and hold shift whilst drawing the line it will create a straight 1 px line etc. This method of pressing shift whilst you draw to keep the cursor in a straight line also works with a number of the other tools including the eraser and the clone stamp.