The Holga Telephoto lens is another great lens that is compatible with both the 120 and 135 Holgas. It's nifty design enables you to slot it right over the Holga lens.
With this lens on, the subjects that you shoot will have a magnified effect as the lens zooms in at 2.5x. You will find that with this lens the photos that you shoot will have an even larger amount of vignetting in them, not to mention that edges in the photos will become even dreamier than standard Holga shots.
It's one hell'uva fun lens to try out!