Parallax occlusion mapping
Marches a view ray through a heightmap in the fragment shader so a flat surface gains real parallax and self-occlusion without a single extra triangle.
Plain parallax mapping shifts the texture coordinate just once and falls apart at grazing angles. POM instead steps the ray toward the camera through the depth field, sample by sample, finds where it crosses the height profile and shades there — pits hide behind ridges as they should and the surface moves correctly as you look across it.
Natalya Tatarchuk (ATI) popularized the technique at I3D 2006. It costs several texture reads per pixel, so it is dearer than normal or parallax mapping but far cheaper than actually tessellating and displacing the geometry. Self-shadowing and silhouette correction can be bolted on as extra passes.