The old video feedback thing: point your camcorder at a screen which is connected to the camera. Trippy!
I haven’t seen this quality anywhere else, though¹. And I haven’t been able to reproduce the unique (?) setup I had back in 2005; a Sony D8 camera (negative effect, slow shutter) and my parents’ 100Hz flat big screen CRT.
¹ Update in 2021: Dave Blair