Solvent Transfers

Doing a solvent transfer is probably the quickest, easiest way to get an image onto a T-Shirt, Moleskine, or other surface. Like screen printing, the image is applied directly the surface. Unlike screen printing, this can be done quickly, cheaply, and without any extra equipment.

You start with a photocopy of the image you want to transfer. Apparently it can be color or black and white. Whether or not inkjet printouts will work is unclear (update: Printouts from an inkjet will not work), but printouts from a laser printer are likely to work. The copy is laid over the surface, face down, and a solvent (xylene or lacquer thinner) is applied to the back. The image is then simply rubbed onto the surface.

The resulting image is a mirrored copy of the original. Simple black and white images such as stencil outlines transfer over very nicely while complex color images will result in a "worn" look (which is all the rage these days).
Xylene works best and can be purchased at your local paint store. You can also use a blender pen from an art supply store (in which xylene is the active ingredient) to achieve this effect. Using a pen works best on smaller transfers while a rag moistened in solvent is suitable for large transfers.
Resources and more information:
Xerox Transfers (Using lacquer thinner)
Blender Pen Transfers
Gallery of Moleskines decorated with this method

Edit: As many people have pointed out in the comments, caution should be used when working with solvents. Always read and follow the guidelines that are printed on any specific solvent and work outdoors or in a well ventilated area (preferably with a respirator). Solvents can pose a serious risk to your health, and we want our readers to continue being not dead.