Free 3D converting and viewing software

MeshLab. The open source system for processing and editing 3D triangular meshes. It provides a set of tools for editing, cleaning, repairing, inspecting, rendering, texturing and converting meshes. It also offers features for processing raw data produced by 3D digitization tools/devices and for preparing models for 3D printing.
OBJ, WRL, OFF, PLY

GLC_Player is a software used to view 3d models (COLLADA 3DXML OBJ 3DS STL OFF COFF Format) and to navigate easily in these models. With the album management, capture and multi-capture capabilities

SketchUp Viewer. Use the SketchUp Viewer for Desktop to view designs in 3D by orbiting, panning, playing animations, and beyond. It’s so simple to use, you’ll impress any client without breaking a sweat. No previous 3D modeling experience required.

Rapidform Explorer is a free viewer for 3D scan data and CAD-models. Open, view and annotate from any supported 3d file format.