Whiteside Corner Chisel
One of the easiest ways to make a mortise is with your router. Unfortunately, making a square inside corner with a round router bit is impossible. That's where a square corner chisel like this one from Whiteside Machine can really help out. A typical chisel doesn't ensure a square corner because it takes two strikes. With this square corner chisel, you get a perfectly perpendicular corner. It's a big improvement.
On the subject of quality, this isn't an expensive tool to begin with. Although there are a number of really cheap imports, this one isn't. It's made by Whiteside Machine, the same folks who make the popular industrial grade router bits. Made in U.S.A.