Photographer moominsean put together a great two-part tutorial/explanation of what light leaks are and how to avoid them or get them on purpose. (See the bright bits coming down from the top on his [utterly brilliant] photo above? That’s them!)
Part one deals with leaks caused by the camera—very common with toy and home-built cameras—while part two deals with the winding of medium format film, which may be of particular interest to Zero Image pinhole shooters.
You can also create lightleaks on film after you have shot the roll (or, theoretically, before, but I haven’t tried this). When you see really strong, overpowering lightleaks, chances are it’s not the camera, but the handling of the film…
Check them out at his new blog, moominstuff: part 1 – part 2