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LDraw™ is an open standard for LEGO CAD programs that allow the user to create virtual LEGO models and scenes. Visit the official LDraw website at http://www.ldraw.org.
LDCad is a multiplatform LDraw editor that lets you edit LDraw model documents in real-time. Visit the official LDCad website at http://www.melkert.net/LDCad.