If you still use 35mm cameras, you can get a really interesting effect by stacking negatives on top of each other when developing the film. The rest of us digital camera slobs can easily recreate the effect in Photoshop.
Photography weblog PetaPixel highlights the stacked negative photography of Laina Briedis, which combines photos of people with images of stars or cloudy skies to produce pictures that look like dreamscapes. If you use film cameras and have access to a darkroom, you can do this easily by exposing two negatives at the same time on the same enlarger.
With digital images, you can superimpose one image on top of another and erase part of one of the layers so the other image shows through and recreates the film technique. You'll also want to play with the opacity levels of the layers to achieve the best effect.