If this innovative little tool didn't already exist, I could see it doing gangbusters on Kickstarter.
When routing out the mortise for a hinge, you of course have the issue that the router bit is cylindrical and thus the corners of the mortise have radiuses. No problem if you're using leaf hinges with rounded corners, but if you're using sharp-cornered hinges, you've got to chisel out the corners of the mortise. Thus Rockler sells this Spring-Loaded Corner Chisel, which registers against the sidewalls and makes the 90-degree downward cut with one tap from the hammer:
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Here's how it looks in action (this isn't the Rockler-branded one, but this Dutch version by 24-7 Wood Easy Solutions appears identical):
Sure you've still got to pare out the waste, and the purist craftsman would probably scoff; but for the tradesperson who needs to bang out a lot of hinge mortises in a hurry, the $27/€20 tool looks like it would pay for itself in no time.