Qfusion is a 3D rendering engine based on sources from Quake II code. It is available for download and modification for free under the GNU General Public License (GPL). The project was started and is primarily developed by Victor Luchitz. It was programmed in C for use on all Windows and Unix-based computers. Qfusion has helped to set the bar for many other Quake II engine projects. A popular feature of the engine is that it is said to be able to correctly render the entire range of content for Quake III. More recent versions of Qfusion engine also extend the Q3 rendering scheme to include per pixel shading support, soft shadows, other newer generation features.
The quake fusion engine can conquer with a lot of common engines these days...and beat them.
The engine includes great improvements in lighting, it also includes dynamic lighting.
Other great things are defractions and refractions, per-pixel lighting (!), relief mapping and more.
The only problem is that the engine has problems with big polycounts, but hey, which engine doesn't have this?