BRL-CAD is a powerful constructive solid geometry solid modeling system that includes an interactive geometry editor, ray-tracing support for rendering and geometric analysis, path-tracing for realistic image synthesis, network distributed framebuffer support, and image and signal-processing tools.
Ayam is a free 3D modelling environment for the RenderMan interface. It features good RIB export, CSG (all RenderMan primitives supported), and NURBS modelling. Features instancing, arbitrary number of modeling views, object clipboard, independent property clipboard, n-level undo, console, and Tree-View with Drag-and-Drop support.
G3D is a C++ library for game developers, researchers, and students. It is a base of robust and high performance code common to most 3D projects. It is not a stand-alone graphics or game engine, but is a set of pieces to jump start your own engine or graphics project. The library has two parts: G3D for pure math, and GLG3D for interacting with hardware acceleration via OpenGL. You can use G3D without GLG3D if you want to use another 3D API (like DirectX) or don't like the OpenGL abstraction.
Brook for GPUs is a compiler and runtime implementation of the Brook stream programming language for modern graphics hardware. The goals for this project are to demonstrate general purpose programing on GPUs, to provide a useful tool for developers who want to run applications on GPUs, and to research the stream language programming model, streaming applications, and system implementations.
Visualization Library is C++ middleware for high-performance 2D and 3D graphics applications based on the industry standard OpenGL 1.x-4.x, designed to develop portable applications for the Windows, Linux and Mac OS X operating systems. It supports advanced features like OpenGL Shading Language, Volume Rendering, Isosurface extraction, Frame Buffer Objects, Multiple Render Targets, Vertex and Pixel Buffer Objects, KdTree/AABB frustum culling, a multilingual Unicode-based text engine, advanced texturing, DDS cubemaps, mipmaps, compressed textures, and much more. Visualization Library can be especially useful in 3D/2D scientific visualization, virtual reality, augmented reality, visual simulation, data visualization, presentations, multimedia applications, special effects, and 3D/2D games.