Every now and then I like to load up the Holga with some 35mm film and shoot - since the image circle cast by the lens is meant for 120 film, it exposes all of the smaller film. The sprocket holes show. You either like it or hate it, I guess, and it's probably an effect best enjoyed in moderation.
You can buy a 35mm Holga, but if you buy the 120 version you get 35mm for free! Just follow the instructions at Squarefrog.