https://blendermarket.com/products/angle-tool

Angle Tool is a simple and robust modeling tool for quickly creating corner geometry from any cross-section selection in Edit Mode. After selecting part of your mesh in Edit Mode and specifying an Active Edge, you can now use the Angle Tool command to turn your geometry around that edge and extend it to create a geometric corner. It's really simple but kind of hard to explain, so please refer to the following visuals :)