(1) Blender
Blender is the open source software for 3D modeling, animation, rendering, post-production, interactive creation and playback. Available for all major operating systems under the GNU General Public License.
(2) G3D Engine
G3D is a commercial-grade 3D Engine available as Open Source (BSD License). It is used in games, tech demos, research papers, military simulators, and university courses. It can support real-time rendering, off-line rendering, back-end game server management of 3D worlds, and use of graphics hardware for general purpose computing.
G3D provides a set of routines and structures so common that they are needed in almost every graphics program. It makes low-level libraries like OpenGL and sockets easier to use without limiting functionality or performance. G3D gives you a rock-solid, highly optimized base from which to build your application.
G3D does not contain scene graph or GUI routines. Because of this, there is a lot of flexibility to how you structure your programs. The tradeoff is that you have to know more about 3D programming. G3D is intended for users who are already familiar with C++ and DirectX or OpenGL. G3D does not draw widgets by itself. Several good G3D-compatible GUI libraries exist and we recommend using one with G3D if your project has extensive UI needs.
(3) OGRE Engine
OGRE v1.2 [Dagon] represents the culmination of 5 years of continuous development, resulting in what is now regarded by many as the leading open source real time 3D rendering engine. OGRE is packed with features to make your development life easier, whether you're making games, architectural visualisation, simulations, or anything else which requires a top-notch 3D rendering solution.
(4) SDL
Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer. It is used by MPEG playback software, emulators, and many popular games, including the award winning Linux port of "Civilization: Call To Power."
SDL supports Linux, Windows, Windows CE, BeOS, MacOS, Mac OS X, FreeBSD, NetBSD, OpenBSD, BSD/OS, Solaris, IRIX, and QNX. The code contains support for AmigaOS, Dreamcast, Atari, AIX, OSF/Tru64, RISC OS, SymbianOS, and OS/2, but these are not officially supported.
SDL is written in C, but works with C++ natively, and has bindings to several other languages, including Ada, C#, Eiffel, Erlang, Euphoria, Guile, Haskell, Java, Lisp, Lua, ML, Objective C, Pascal, Perl, PHP, Pike, Pliant, Python, Ruby, and Smalltalk.
SDL is distributed under GNU LGPL version 2. This license allows you to use SDL freely in commercial programs as long as you link with the dynamic library.
(5) Sandy
Sandy is an intuitive and user-friendly 3D open-source library developed in Actionscript 2.0 for the Flash environment.