www.OpeningDesign.com is basically Github/GradCAD for Architects and Engineers (building/construction industry).
That is, a social platform for architects/engineers to share digital content (BIM/CAD) via web-based, real-time collaboration tools.
--SketchSpace_2D: a web-based tool, allowing construction professionals to mark-up and sketch on blueprints in real-time.
--(future) SketchSpace_3D: a web-based BIM/CAD viewer.
Exec. Summary: http://bit.ly/11ZTkWj