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. We've got a huge active community, and there are lots of projects in development that use the engine. You can find enhancements for Irrlicht all over the web, like alternative terrain renderers, portal renderers, exporters, world layers, tutorials, editors, language bindings for java, perl, ruby, basic, python, lua, and so on. And best of all: It's completely free.
Mostly source code - "Software development kit required to develop applications with the Irrlicht engine. Included is the documentation, the source code and a pre-compiled version of the engine, 19 tutorials, some useful tools, exporters, an interactive demo which shows the capabilities of the engine and more."