Omnidirectional shadow maps
Shadow maps for point lights, which cast shadows in every direction — usually solved by rendering depth into a six-faced cube map.
An ordinary shadow map is enough for directional and spot lights, which shine one way: you render the scene's depth from the light's view once. A point light shines in all directions, so it needs shadows across the whole sphere around it. The standard solution is a cube map — the scene depth is rendered six times, one per cube face — usually storing the radial distance to the light rather than clip-space depth.
The cost is six render passes per point light, which adds up fast. Optimisations include single-pass layered rendering with a geometry shader, dual-paraboloid maps that make do with two views, and refreshing only lights that have moved. The technique carries the same weaknesses as all shadow mapping: resolution stair-stepping, shadow acne and peter panning.