When you line up a cut with your circular saw, you’re always trying to eye exactly where the blade’s going to cut. And if you use a guide, you have to be sure of the exact distance between the saw’s guide and the blade.
The Pro-Cut is a simple plastic guide, but what makes it really slick is that when you make your first cut with it, you trim away the edge of the tool instantly aligning it permanently with your specific saw. From then on, making a 90 crosscut is as simple as placing the edge of the tool using your measuring tape (its multiple guides automatically align it to 90 degrees on various common-dimensioned lumber) and cut away.
It also includes a built-in 6″ ruler and 45 degree triangle for measuring and layout work. Street pricing starts around $15, and some distributors offer multi-packs.