Reflective shadow maps (RSM)
A shadow map reused as a light source: every lit texel becomes a tiny secondary lamp that casts indirect light.
The technique was presented by Dachsbacher and Stamminger in 2005. An ordinary shadow map stores only depth; an RSM also stores world position, normal and flux (coloured light) for each texel. Each such texel is treated as a virtual point light, and by sampling a handful of nearby texels you get one bounce of indirect light almost for free.
The limitation is that it is a single bounce, and the virtual lights cast no shadows of their own — there is no indirect occlusion, so light can leak through walls. Even so, the method is the foundation for several real-time GI techniques, among them light propagation volumes.