Section Properties Plugin

 Here's a new engineering plugin by engineer and SketchUp user Osvaldo Salas. There is both a limited Free version and a Pro version which is $26. The Pro version includes more functionality, a toolbar, and reporting.

SkpEngineering: Section Properties Calculator is an engineering plug-in designed for SketchUp. The plug-in locates the centroid, plastic neutral axis and calculates mechanical properties of any closed planar face defined in SketchUp. Some common applications of the calculated properties include structural design of aluminum extrusions, steel, wood and light gauge metal steel members.