Wallboarder's Buddy Drywall Trimmer by Wallboarder's Buddy Tool Co.
Only a professional builder could've invented this, but we're pretty sure that all DIYers can rejoice. A T-square fitted with a dual-sided, spring-loaded utility knife replaces the measuring, squaring, and chalking of each cut. Just set the knife to your width and pull the square along the factory edge, reverse, then repeat on the opposite face.
About $60; wallboardersbuddy.com