In photography and cinematography, multiple exposure is a technique in which the camera shutter is opened more than once to expose the film multiple times, usually to different images. The resulting image contains the subsequent image/s superimposed over the original. The technique is sometimes used as an artistic visual effect and can be used to create ghostly images or to add people and objects to a scene that were not originally there. It is frequently used in photographic hoaxes.( Wiki )

And here are some pretty awesome multiple exposures from Flickr. Most (if not all) of these were done with film.

V&A vs NH 6 by 25ThC

Roadside Reflection by wheehamx

Feria de abril by killer blythes

Will you be there when the sun goes down? by new account

Untitled by Tamara Lichtenstein

Agitation by y2-hiro

Untitled by -Antoine-

Multiple Hits on Target by bigdaddyhame

Moonrise Over Bridalveil Falls by Paul Porter Photography

Game On by Ningaloo

In Limbo by Rob bix

Untitled by liamhart

Bellevue DRI by Bryan Koorstad

last light by Katharina Becker

embassy falling by microabi

I I I I by scott w. h. young

Untitled by mister sullivan

alabaster by .ultraviolett

Summer Clouds by DomiKetu

Winter Sleep by Tamar Burduli

Tara Tree by Juan Ordonez

Untitled by emilyharriet

Lysergic Icarus by supermarketsong

Lifeguard-Towering-Silhouetted-Sunset Flip by roostercoupon

. by shutterback

Double telephone by RL Stars

ulisses by Fábio Lamounier

Nanga Parbat by Usman

triple exposure with minor light leak by vivek jena

11/20/11 by Photon Thief

Stairs by Groovnick

Ken in Motion by HE~GOES

236/365 by aaaamandaaaa

Untitled by victoriahhhh

Untitled by Edward Cooke