Libraries and source-code

Ogre 3D (C++, Direct3D/OpenGL)

GLScene (Delhpi, OpenGL)

Irrlicht (C++ / .NET, Direct3D/OpenGL)

Brume (C#, DirectX)

AdVantage Terrain Library (C++)

Panda3D (C++ / Python, OpenGL)

Ca3D-Engine (C++, OpenGL/DirectX)

Multiverse (MMO engine)

Torque 3D Engine, by GarageGames (C++, DirectX).

Truevision3D (C++, VB.NET, C#, other languages, DirectX).

Blitz3D (BASIC, DirectX).

C4 Engine, by Terathon (C++).

Unity game engine and editor (no code).

Leadwerks engine (C, C++, BlitzMax, C# and Delphi).

SunBurn engine, by Synapse Gaming (XNA/.NET, DirectX).

Nuclear Basic (BASIC) and Nuclear Fusion (C++), by Nuclear Glory (DirectX)

See libraries used by L3DT.

See The Code Project.

See CodeGuru.