Calligrams
Calligrams are a a combination of an image and a field of text. In order to build a calligram, you need to create a displacement file (showing the contours of the original image). Then, you revert to your original image and add your text layer and apply the displacement file that you ha already created. In essence, you are creating a mask that only records the image in the text spaces. This merge is then pulled out to a separate file and placed on a black (or other color) backdrop. Here are a couple of samples; one black and white and one in color.
Dispersion Effect
This is a cool use of two different kinds of masks, one on top of the other.
This process also makes use of the liquify tool in photoshop.