Also known as “Orton imagery,” the technique invented by stock photographer Michael Orton in the 1970s. He used a tripod to take two slide photos of a scene, one underexposed and another overexposed and blurry; by superimposing the two slides, the resultant image gave the scene a glowing, ethereal, “otherworldly” effect.
Chris Empey’s open source tutorial for emulating the Orton technique. [original]
Another Gimp tutorial.
There are also plugins, such as the orton effect plugin [example] but I’ve found, as with the fakelomo plugins, the hands-on image modifcation you can do yourself is better than the plugins almost all of the time.