Last Wednesday, Ken Silverman released a demo of Build2, an engine he worked on from 2006-2011.
https://youtu.be/3qtmkkdND6M
EDIT: How do I manually embed videos?
Features of the engine include:
Quote
Native Windows, 32-bit color, 6 degrees of freedom, pure CPU rendering
Native support for sector over sector (SOS).
Advanced lighting system with true dynamic shadows, colors, spotlights.
Multi-user editing with client-side prediction.
Powerful scripting compiler in EVALDRAW.
Full RGB color mapping.
Voxel sprite support.
Skybox support.
No sector/wall/sprite count limits.
Native support for sector over sector (SOS).
Advanced lighting system with true dynamic shadows, colors, spotlights.
Multi-user editing with client-side prediction.
Powerful scripting compiler in EVALDRAW.
Full RGB color mapping.
Voxel sprite support.
Skybox support.
No sector/wall/sprite count limits.
It is also missing some features from the original Build, such as transparency and built-in texture animations.