LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
Floorplanner - Floor plan interior design software. Design your house, home, room, apartment, kitchen, bathroom, bedroom, office or classroom online for free or sell real estate better with interactive 2D and 3D floorplans.
FreeCAD - An open-source parametric 3D modeler
Sweet Home 3D - Sweet Home 3D is a free interior design application that helps to draw the plan of house, arrange furniture on it and visit the results in 3D.
gCAD3D - 3D - CADCAM - Software for MS-Windows and Linux. Freeware
SketchUp - 3D for Everyone