Dual Worlds by Ryan Dehaven
An implicit world engine that uses ideas from the dual contouring paper presented here. The engine supports different materials for the implicit object and negative space material. The engine generates polygons from scene with implicit data. Any implicit data with a inside or outside test can be supported. Instead of a QEF approximation, a average of the intersection points is taken for each cell around the object. Objects are divided into cells using a subdivided structure called an octree.

Negative material example:
sphere slice
Two sphere merging together:
two balls