3DE is a freeware 3D-Engine with the following features:
Multiplayer 3D-Ego-Shooter
Map-Editor which lets you create maps directly in a 3D-Environment (there are no additional 2D windows necessary)
Support for "Quake", "Quake 2" and "Quake III Arena" based maps (this also includes "Half-Life", "Kingpin", "Voyager: Elite-Force", "Return to Castle Wolfenstein" and more)
Support for uncompiled maps created with "GtkRadiant", "QERadiant", "Q3Radiant" etc.
Full source code
English and German language versions
HTML Documentation (English and German)


New Features:

Model support
3DS, Quake 3 MD3 and DirectX file-support

Rendering optimizations
Octree & PVS (Pre-calculated hidden surface removal)
Hardware vertex buffers etc.

Dynamic Textures
Shader script language that allows the use of bump maps, environment maps etc.

Gameplay
Animations

The source code
Perfectly structured (no longer hundreds of global variables with strange names)