Virtools the Behavior Company  
   Tools for the development of 3D graphics. Primarily for use with development of online applications, such as e-Marketing and e-Learning, as well as games.
   http://www.virtools.com/

Bolt3D  
   A free real-time 3D graphics library for Windows with features such as gouraud shading, phong shading, texture mapping, and particle systems.
   http://www.geocities.com/SiliconValley/Way/7233/

Obsisian  
   An open source 3D virtual world for Linux and OpenGL.
   http://www.zog.net.au/computers/obsidian/

Quesa  
   A free (LGPL-ed) and open source, high level 3D graphics library which offers binary and source level compatibility with Apple's QuickDraw 3D API.
   http://quesa.designcommunity.com/

GNU 3DKit - Free Software Foundation (FSF)  
   The GNU 3DKit provides several frameworks to render 3D graphics in a GNUstep environment. Its main purpose is to render any kind of 3D graphics in real-time on commodity hardware, using an optimised scene graph architecture.
   http://www.fsf.org/software/gnu3dkit/gnu3dkit.html

PLIB Home Page  
   Portable Software using Portable Libraries. PLIB is a set of libraries that are designed to help developers of interactive 3D applications such as games. All PLIB libraries are portable to just about every modern Operating System.
   http://plib.sourceforge.net/

Expression 3D Toolkit  
   A free Object Oriented 3D toolkit. It is written in C++/STL. It has support for interactive 3D graphics using OpenGL and non-interactive sphere tracing.
   http://www.kotterink.com/expression

Gizmo 3D  
   Site for Gizmo3D, a high performance scene graph 3D engine.
   http://www.tooltech-software.com/

3Dgpl  
   A 3D Graphics, Portable Library which provides basic set of tools for writing 3D applications, games for instance.
   http://www.gamers.org/dEngine/rsc/3dgpl/

Glide for UNIX  
   A project to support 3Dfx Glide API on Posix OS.
   http://www.alasir.com/glide/

OpenSG  
   A portable C++ scene graph library for 3D graphics applications. Its main distinguishing features are extensive support for multi-threading, reflectivity and good extensibility. [Open source, LGPL]
   http://www.opensg.org/

Velocity 3D  
   Source code for an entirely software 3D-engine and a DirectX 3D-engine, along with screen shots, downloads and documentation.
   http://www.razorforce.co.uk/dev/

Read3DS  
   A 3D Studio file (*.3DS) reader and C++ X11 3d engine for Unix. [Freeware]
   http://pascal.leynaud.free.fr/

OpenNPAR  
   Provide a unifying framework for the creation of Non-Photorealistic Animation and Rendering. Images, videos, publications and demos available.
   http://www.opennpar.org/

VCGLib: A Visualization and Computer Graphics Library  
   A portable C++ templated library for efficient manipulation, processing and displaying with OpenGL of triangle and tetrahedral meshes.
   http://vcg.sourceforge.net/