Arbaro the trees generator - we all know how expensive these kind of programs are and here you have a free version coming from the open and free world.-Povray, DXF or Wavefront OBJ files.

Aqsis - " is a high quality, photorealistic, 3D rendering solution. It complies with the RenderMan® interface standard defined by Pixar. The RenderMan® standard has been used in film and television visual effects since it's introduction in 1989. Pixar has used their own implementation for all of their award winning CG features, and provided their implementation for use in the visual effects of most major blockbuster films over the last 2 decades. The Aqsis project offers a way for individuals and organizations alike to gain experience with the RenderMan® interface without the cost of commercial software licenses. Aqsis comprises a command line rendering tool, a tool for compiling shaders in the RSL language, a tool for preparing textures for optimal use, and various developer libraries to enable integration with third party tools. Aqsis is licensed under the GPL license, with some parts under the LGPL. "

Inkscape - this is a great piece of free software (open to be modified and at no price) to edit and create vector images with advanced features. Ok, this is not a copy of, say, Adobe Illustrator®, but it is a serious program to be used in a wide range of situations. Inksacape is ready for GNU/Linux, Windows® and Macintosh®. Some features: calligraphic strokes, Text tool, support imported linked images, tablet support (angle and pressure), layers (as everybody know them), Masks and clipping paths, PDF export, and a very long etc.

Pixie - This is a renderer that produces photorealistic images. Pixie is open source software and is based on RenderMan. It's available for Linux, MacOSX and Windows. This project is licensed under GNU Lesser General Public License (LGPL).

K-3D - " is a complete free-as-in-freedom 3D modeling, animation and rendering system. K-3D features a robust plugin architecture and visualization pipeline, designed to scale to the needs of professional artists. K-3D is designed from-the-ground-up to generate motion-picture-quality animation using RenderMan-compliant render engines." K-3D is mainly developed for GNU/Linux, but you can find also a version for Mac OSX, and Windows XP. This project started in 1994 with Tim Shead and was called Equus-3D. Some features are: K-3D is a procedural modeler (woa!), unlimited undo/redo for anything, supports Aqsis (see Aqsis above) and Renderman® to render , real time OpenGL solid, shaded and texture-mapped visualizations, etc.

YafRay " is a powerful raytracer, under the LGPL license. It enables you to create fantastic images and animations of a photorealistic quality." Some of the most relevant features : Full Global Illumination, Skydome Illumination, HDRI Illumination, Caustics, Glossy/Blurry reflections, Modular framework, Integration in 3d suites via plug-in, Platform independent (fully developed using C++), Distributed and multithread render, etc.

MakeHuman " is a completely free, innovative and professional software for the modelling of 3-Dimensional characters. The features that make this software unique of its kind are the new Tetra-parametric GUI© components and the Natural Pose System©, for advanced muscular simulation. Using MakeHuman a photorealistic character can be modeled in less then 2 minutes; MakeHuman is released under Open Source Licence, and is available for all the platforms (Windows, MacOsX, Linux) (...)" this program uses the Renderman standard also.

Gimp is an image manipulation and creation program, like Photoshop®, on which you can rely because it is a professional software. Some features: vectors, layers, advanced and professional image editing, color manipulation, support for many image formats, animations, scripts, advanced brush creation, is open software so you can adapt it to your personal needings (you can modify it if you are a programmer), etc. Its name means 'GNU Image Manipulation Program' and is a completely free piece of software. (yes, free. you don't have to pay for it). You can find Gimp for Macintosh®, Windows®, GNU/Linux. There are also: GIMP Portable, which is a version that can be run from a portable media, like an USB memory, with no installation needed. Seashore, which is a Macintosh only version with a native interface. GIMPshop, that tries to imitate the very Photoshop® interface. There's also something known as Film Gimp, which actually is CinePaint, to be used as frame-by-frame retouching.

Blender is a complete and professional 3D content creation program available for all mayor operating systems (GNU/Linux, Macintosh® -PPC and Intel, Windows®, Irix® and Solaris®). It's free (as in 'no money') and open to programmers. There are several free 3D programs already but Blender is the most complete and popular among them. Its feature list is impressive and here you have some of them: several types of primitives forms, as in FormZ®, like Nurbs primitives, polygonal and Nurbs meshes, Metaballs, etc., many formats support, inverse cinematics, fast skeleton creation, scripting, animation, particles, fast inbuilt raytracer, subdivision surfaces (like in Cinema 4D®, Lightwave®, etc) , multiresolution sculpting (!!woa!!), realtime 3D game creation, etc.