The chalk line: simple, effective.

The trouble with most putting training aids is that they may not work for every type of stroke. Swinging door, or pendulum? But my favorite is the chalk line, because it works for every type of stroke, and offers direct, immediate feedback. Simply find a straight putt, clip the end of the chalk line over the edge of the hole, and snap the line to make a 3 or 4 foot straight line on the grass. Then start rolling your putts down the line.