3D Engines

APOCALYX Engine
OpenGL 3D engine with GPL license, 3D, Physics and AI.
http://apocalyx.sourceforge.net/

Bonzai Software
3D game engine based on the OpenGL API written in C++. Also real-time computer graphics examples using the OpenGL API.
http://www.bonzaisoftware.com

Crystal Space
Free (LGPL) and portable 3D Game Development Kit written in C++, OpenGL and software renderer.
http://www.crystalspace3d.org/main/Main_Page

3D Engines Database
Overview of 3D engines for games, business applications and virtual reality with various platforms, APIs and programming languages.
http://www.devmaster.net/engines/

EGLE
The Enhanced OpenGL Engine (EGLE) is a freeware and open source engine based on OpenGL that uses Quake .map and .lmp files.
http://www.geocities.com/SiliconValley/Lakes/7220/egle.html

ExoEngine
C# OpenGL and Cg 3D Game Engine for .NET
http://www.exocortex.org/3dengine

GLScene
Delphi OpenGL engine including visual components and objects for rendering 3D scenes.
http://glscene.sourceforge.net/

The Nebula Device 2
Realtime 3D game/visualization engine, written in C++, scriptable through Tcl/Tk, Python and Lua. Supports D3D and OpenGL for rendering, runs under Linux and Windows. Includes news, documentation and downloads.
http://nebuladevice.cubik.org/

OGRE
C++ Object-oriented Graphics Rendering Engine.
http://www.ogre3d.org/

Titan Project
A game engine using OpenGL that is able to render Quake3 maps and supports most Quake3 features. [Modified BSD license]
http://castano.ludicon.com/oldies.php

Universe Generator System
Full featured, object oriented and multiplatform 3D engine.
http://ugs3d.sourceforge.net/