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.
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.
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.
Dice3DS is a set of Pure Python modules for dealing with 3D Studio format (*.3ds) files. It maps 3DS chunks into a hierarchical set of Python objects; it's a lot like how ElementTree works for XML files. Dice3DS requires Python 2.6 or higher and Numpy. Note that it is not a wrapper for lib3ds.
FreeSG is a portable, high-level 3D engine with an emphasis on generality. It is designed as a component to be integrated, as opposed to an application framework. Its goal is to support applications ranging from small 3D games running on portable consoles to large simulations that must be rendered photorealistically using fourth-generation graphics hardware.
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.