Open Source STL Viewers
Linux Mac Windows
GLC_Player
GLC_Player is an Open Source software used to view 3D models (COLLADA 3DXML OBJ 3DS STL OFF COFF Format) and to navigate easily in these models. English and French interface.
Linux Mac Windows
Gmsh
Gmsh is a 3D finite element grid generator with a build-in CAD engine and post-processor.
Linux Mac Windows
Meshlab
http://meshlab.sourceforge.net/
Besides being a viewer, this is also a very powerful mesh editor. Meshlab supports a very wide range of 3D file formats.
Linux Windows
Viewstl
http://sourceforge.net/projects/viewstl/
Viewstl is an open source way to view Stereo Lithography Files as shaded on-screen images. Ascii STL files and dynamic rotation, scaling, and panning are currently supported.
Free STL Viewers
Windows
EasyViewStl
http://www.gcad3d.org/EasyViewStl.htm
Simple but lightweight and fast viewer.
Unfortunately only available with German interface
Windows
MiniMagics
http://software.materialise.com/minimagics
MiniMagics can also analyze and section parts. English, German and Japanese versions.
Linux Mac Windows
NetFabb Studio
Powerful viewer that can also repair, scale, cut, edit meshes and much more. English and German interface.
Windows
VisCAM View
http://www.materialise.de/fusion-von-marcam-engineering-und-materialise-gmbh
VisCAM View is a very easy-to-use and efficient tool to view, check, measure and communicate STL files. Visualisation of STL, 3DS, VRML, DXF, PLY, ZPR and VFX files.