LibreCAD - An open source 2D CAD application for Windows, Apple and Linux.
STP Viewer - Free STP Viewer - free software that lets you view 3D STEP files (.stp, .step file format).
FreeCAD - An open-source parametric 3D modeler
A360 VIEWER - Free online CAD Viewer
SketchUp - 3D for Everyone
ODA Viewer - ODA Viewer is a web-based software solution that allows you to view the files based on ODA format and you can easily zoom the file for detail visualization.