The best free alternative to 3D Viewer Online is Online 3D Viewer, which is also Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to 3D Viewer Online and eight of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to 3D Viewer Online are STP Viewer (Free), A360 VIEWER (Free), CAOdrive (Free) and VariCAD Viewer (Free).
A360 Viewer is a free online viewer for 2D and 3D designs including AutoCAD DWG, DXF, Revit RVT and Inventor IPT, as well as STEP, SolidWorks, CATIA and others. These file types can be viewed with the online A360 viewer, the free version of A360, and the paid version, A360 Team.
Component of IDA-STEP providing core viewing functionality for all kinds of STEP files (*.stp). This component alone makes a fully usable tool for viewing PDM, CAD and other data, available as STEP (internation standard ISO 10303) files.