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FreeCAD - An open-source parametric 3D modeler
BIM Vision - Freeware IFC model viewer.
LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
FZKViewer - FZKViewer visualises the sematic data models IFC, CityGML und gbXML
SketchUp - 3D for Everyone
usBIM.viewer+ - Do you need to open an IFC file and modify it? usBIM.viewer+ is the only freeware certified by buildingSMART International that allows you to view, edit, convert and share BIM models in the IFC format. Basically, it is more than just a viewer!