A 4ft ruler from your local hardware store. The yellow ones are best as one side is blank and even comes with a 1/4″ hole pre-drilled at the end.
Place your golf ball on the drilled hole and putt down the ruler at various speeds on a level surface. If your golf ball makes it all the way down the ruler, you know your club face is within 1 degree of square at impact. This is one of the best training aids to work on the start-line of your putts.
Other benefits of the ruler include it fitting right next to your driver in your golf bag, and the fact it can be used indoors or out on a real putting green. Best $4 training aid in golf.