Straighten Curve Tool

This tool makes sections of a curve into straight lines, useful for making machined looking objects

When you pick the Straighten Curve Tool from the Tools Panel the tool cursor changes to .

Straighten Part of a Curve

Select a curve, (Drag with the RMB until a curve is highlighted in yellow)
Then drag between two points, starting and ending on the curve, with the LMB.
When you let go of the LMB the curve straightens between those two points, and the object is rebuilt.