there is PlanCAD-L as freeware in Dietrich's download section. You can share plans in Dietrich's format, your client can add remarks and send them back to you.
If you want to share 3d models, there are export formats (optional): 3d-dxf or vrml. In case you have Adobe Acrobat 3D (no low cost prog!), you can create 3d-pdf to share them.
