NVIDIA Gelato
NVIDIA Gelato rendering software brings high-quality rendering to the masses. A hardware-accelerated, non-real-time renderer, Gelato allows you to use your NVIDIA GPU to create stunning images fast. Originally designed for the creation of 3D visual effects and animation in feature films, Gelato can be used by anyone with NVIDIA hardware to create images of the very highest quality. Gelato is a software program that leverages the NVIDIA GPU as a floating point math processor. This allows Gelato to render images faster than comparable renderers, but without the quality limitations traditionally associated with real-time graphics processing on the GPU. Yes, Gelato is available free of charge. Now students, freelancers, startups, and small studios can take advantage of a high-end renderer with the capabilities of renderers costing thousands of dollars.

Sunflow
Sunflow is an open source rendering system for photo-realistic image synthesis. It is written in Java and built around a flexible ray tracing core and an extensible object-oriented design. It was created as a framework for experimenting with global illumination algorithms and new surface models.

Kerkythea Rendering System
Kerkythea is based on the source code I developed from scratch in the past for various applications including C++ containers, numerical analysis, color management. Nevertheless, it uses some popular external libraries as well mostly for window and image management. Apart for the MFC used in the windows standard interface (something that is going also to be replaced shortly), all these libraries are open source (although K is released without source).

Shoran Software RenderPal 2004 Workstation
RenderPal 2004 Workstation is an exceptionally feature-rich and easy-to-use solution for batch rendering on a single workstation for various renderers, includung Maya, Mental Ray, XSI, Lightwave and more. Its intuitive and modern concepts allow everyone, whether hobbyist or professional, to render scenes with ease. Offering the ability to change a great variety of render settings on-the-fly, queuing up of multiple scene files and an advanced job queue, RenderPal 2004 Workstation is an invaluable application for every production.

POV-Ray.org The Persistence of Vision Raytracer
The Persistence of Vision Raytraceror or POV-Ray for short, is a high-quality, totally free tool for creating stunning three-dimensional graphics. It is available in official versions for Windows, Mac OS/Mac OS X and i86 Linux. The source code is available for those wanting to do their own ports.

DrQueue
DrQueue is an Open Source render farm managing software. It distributes shell based tasks such as rendering images on a per frame basis. DrQueue works under Linux, FreeBSD, Irix, Mac OS X and Windows. It is distributed under GPL and is composed by three main tools: master, slave and drqman.

3Delight
3Delight is a fast renderer that supports many of the features required to produce photorealistic images, including motion blur, depth of field, programmable shaders, antialiasing, ray tracing and the ability to read RIB (RenderMan) files. It is currently available for the MacOS X, IRIX, Linux-i686, LinuxPPC and Windows platforms.

VIRTUALIGHT Renderer
VIRTUALIGHT is a sophisticated standalone rendering engine that allows previewing and photorealistic rendering of 3D scenes. Like the RenderMan Interface (R) from Pixar, its interface may be thought of as a 3D scene description format (this interface is not Renderman compliant). It can be used as a communication protocol between any modeling programs or production packages and the VIRTUALIGHT renderer to produce photorealistic quality images. VIRTUALIGHT is free for personal use and available for Windows (Win32). A Linux version should be coming soon. However, it is a shareware software. This means that to keep using it for educational purpose or commercial gain of any kind (including profiting from any published images or animations created with it), you will be requested to register by sending the author a donation ($50 US).

Jacopo Pantaleoni Lightflow
The Lightflow Rendering Interface is an object oriented programming interface to synthetic rendering, which provides a common environment and description language to treat light distribution in three dimensional spaces. A major advantage of the system is that it has been made looking for complete extensibility since its creation, providing the user with simple and flexible extension mechanisms, which allow, among the others, procedural definition of: surface and volumetric patterns and materials, new parametric surface types, general lighting systems, camera lenses and output redirection. The interface is made up by a set of language independent classes and procedures that allow to define three dimensional scenes, so that an actual interface implementation may be easily used both as an output device by external programs and as a separate rendering tool by single individuals. Currently, two versions of the Lightflow Rendering Interface are freely available for non commercial uses: the Client Side C++ API, and a limited Python wrapping that allows easy scene construction via scripting.

Michael Granz WinOSi
WinOSi is an Open Source, Freeware Rendering Tool for creating real photorealistic Images by a new algorithm, which I have called 'Iterative Two Pass Optical Simulation Raytracing'. This algorithm combines the advantages of conventional raytracing and radiosity methods together with stunning new optical effects like caustics, color dispersion and global illumination, which are not covered by standard rendering algorithms.

Aqsis
Aqsis is a suite a tools for producing photorealistic 3D images. Aqsis is Renderman® compliant, that is it conforms to the requirements laid down by the Renderman® specification which is the property of Pixar. This means that it can easily be interfaced to 3D applications which output data compatible with Renderman® based renderers.

YASRT Raytracer
YASRT is a raytracing program. It uses the raytracing algorithm to generate realistic pictures from textual descriptions of three-dimensional scenes. It uses the metaphor of photography to achieve this goal. The photographer (the user of the program) uses a camera (the program) to shoot (render) photographs (two-dimensional pictures) of the surrounding world (text description of a three-dimensional scene). By using advanced raytracing techniques, YASRT can simulate realistic optical phenomena: reflection (objects can be reflected in mirrors), refraction (object can be seen through other transparent objects), shadows are cast when the light is blocked by one or more objects, objects have different surface properties, etc. YASRT is free for personal use.

G3D 3D 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 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.

The OGRE Team OGRE 3D rendering engine
OGRE v1.0 is 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.

Irrlicht 3D engine
The Irrlicht Engine is a cross-platform high performance realtime 3D engine written in C++. It is a powerful high level API for creating complete 3D and 2D applications like games or scientific visualizations. It comes with an excellent documentation and integ- rates all the state-of-the-art features for visual representation like dynamic shadows, particle systems, character animation, indoor and outdoor technology, and collision detection. All this is accessible through a well designed C++ interface, which is extremely easy to use.