The Persistence of Vision Raytracer is a high-quality, totally free tool for creating stunning three-dimensional graphics.

The Irrlicht Engine is an open source high performance realtime 3D engine written and usable in C++ and also available for .NET languages. It is completely cross-platform, using D3D, OpenGL and its own software renderer, and has all of the state-of-the-art features which can be found in commercial 3d engines.

GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring.

Many PHP based gallery systems currently exist.  However, many require setting file permissions as writable for the webserver or a connection to a database.  Often it is not possible or practical to do this, especially if you do not need advanced functionality.
FreeGallery is designed to fit into the gap this category, simple copy the necessary files to the webserver, no further setup is needed.

K-3D is free-as-in-freedom 3D modeling and animation software. It features a plugin-oriented procedural engine for all of its content, making K-3D a very versatile and powerful package.
K-3D excels at polygonal modeling, and includes basic tools for NURBS, patches, curves and animation.

Crystal Space is a framework for developing 3D applications written in C++.

The leading Open Source 3D game and visualisation engine used in dozens of commercial games and professional visualisation applications released worldwide.