WETOLS Corner Clamp Set

I used these constantly while building the cabinets in my shop. They hold two pieces together at a 90° angle, which makes it so much easier to keep everything square while you glue, screw, or fasten the pieces together.
 
They were especially helpful when I was working by myself because I didn't need an extra set of hands trying to hold both pieces in position while I attached them. They're inexpensive, simple to use, and one of those little tools that makes building cabinets, boxes, frames, and other woodworking projects way less frustrating.