Bent normals
A per-pixel direction pointing toward where the surrounding geometry is least occluded, used to make ambient lighting respond to nearby surfaces.
Plain ambient occlusion only darkens — it tells you how occluded a point is, but not which way the open sky lies. A bent normal records exactly that direction: the average of all unoccluded rays over the hemisphere. Light the surface with the bent normal instead of the geometric one and a crevice draws its ambient light from the opening rather than from the wall it faces.
They are usually baked offline alongside the AO map (and often packed into the same texture set) and also improve how reflections and specular occlusion read inside cavities. The payoff is subtle, but it removes the flat, uniformly grey look that AO alone gives deep recesses.