looks at the contents of a selected group and tries to fix it so it is 'manifold' for 3D-printing etc.
The group needs to be a 3D shape which is fully surfaced without internal divisions or intraneous/extraneous geometry/entities...
It removes 'non-face/non-edge' entities.
It removes 'disconnected geometry'.
It removes 'unfaced-edges'.
It runs an initial 'health-check' and reports.
It removes face 'flaps'.
It heals 'holes'.
It removes 'internal faces'.
It orients all faces consistently to face 'outwards'.
It erases 'coplanar edges' [optional].
It 'triangulates' all of the faces [optional].
It highlights major errors.
Manifold is available via SketchUcation Forums