Umbra
Finnish occlusion-culling middleware that works out ahead of time what can hide what, so the engine doesn't draw everything behind the walls.
Classic approaches such as BSP trees and PVS assume the level is built from sealed rooms and corridors. Umbra, from the Helsinki company of the same name, accepts arbitrary geometry instead, bakes it at build time into a compact spatial database, and at runtime answers "what is visible from here?" with a fast software rasteriser on the CPU — no waiting around for GPU occlusion queries.
Its most widespread deployment is oddly anonymous: Unity's built-in occlusion culling is Umbra under the hood. Anyone who has pressed Bake in the Occlusion window has been running Finnish middleware.