FlashPolygons allows to create closed polylines from a selection with AutoCAD.
Editor: FlashPolygons allows to Create closed polylines from a selection of polylines, lines, arcs and circles. The program detects and built all the possible boundaries formed by Entities of the drawing. The result is a set of 2D polylines representing the internal and external boundaries.

It is not necessary that the elements to be treated are perfectly trimmed. FlashPolygons will Detect intersections and will eliminate isolated segments when calculating the polygons.

You can also specify a minimum area below which the polylines will not be created. you can free download FlashPolygons 1.0 now.