What are your favorite photo tricks that can be done entirely in camera?

I love the "ghosting" technique. Long exposures and the brief appearance of an object renders that object translucent. I am always trying to use it for still life.

I've tried light painting with a multi color LED toy. With a long exposure in a dark room, the toy is moved around slowly. The reflections off of the walls and roof are captured by the camera. This has created some very scary images of an otherwise familiar room.

I have also created images with a "hazy" appearance for landscapes and still lifes by simply moving a reflector up and down between the camera and the scene during a long exposure.