A red laser pointer. A chunk of “bad” glass. A blank wall. And here, a remarkable thing.
From the Wikimedia Commons description:
A red laser beam passes through an irregular sheet of glass and produces an image on the wall. Here the caustic effect, normally seen with incoherent light, is combined with interference of laser rays travelling along different lenght paths because of varying refraction angles. Class II laser pointer, 650nm lambda, 0.05m laser to glass distance, 3m glass to wall distance.
You can read more about caustic effects in optics here, and see a few beautiful examples – some much more recognizable than this one – here. Basically, it’s what happens when light passes through a curved, transparent substance and gets focused brighter here, dimmer there.