The Holga is a plastic toy camera made in China. It shoots 6 x 6 cm negatives on medium format (120) film, and costs less than $20. Its many advocates feel that the flaws in the plastic lens and mechanics give its images an appealing look (sort of the visual equivalent of vinyl fetishism). Light leaks, vignetting, loss of sharpness and all sorts of other imperfections might befall your images. See below for a case in point. I still haven't figured out exactly what happened to this exposure, but I like it anyway. (Click image for a larger view.)
On the other hand, due to the cameras simplicity, there are lots of interesting things one can do like create multiple exposuresjust don't advance the film between shots (this will happen whether you want it or not, by the way!) or run frames together by advancing the film incompletely to create panoramic collages. (See examples below.)
See more of my Holga photos on my flickr pages.
The best place to get a Holga is definitely from Randy Smith at Holgamods.com. Randy offers many useful modifications to the stock Holga such as bulb capability, a cable release and pinhole adaptation.
Freestyle Photographic Supplies is a great source for unmodified Holgas (as well as many other toy and novelty cameras from around the world) and Holga accessories such as a Holga flash, Holga bags, a Holga enlarger and a Polaroid Holga back (!).
Susan Bowen has done a lot of interesting work making overlapping image collages using the Holga.