Current Freehand Tool:
Hold shift while drawing freehand lines to smooth them after the mouse button is released.
P.S. I know about the bezier tools and they work excellently, but adding a smoothing option to the freehand tool might be easier for certain situations. For example, drawing artistic linework might be faster with the freehand tool than with the bezier tools. I noticed the freehand tool doesn’t have any modifier keys, so there’s room to add a smoothing modifier.