This nifty little utility is useful when you want to see how your code handles low disk space conditions. DiskFiller allows you to specify how much space you want to leave free on any mounted disk (in bytes or in K), and it creates a "Filler" file that fills the disk, leaving only the specified amount of space free. The "Filler" file is placed on the top level of the filled disk, and to regain the space you can just throw the file out.
The utility doesn't notice if a "Filler" file already exists, so you probably have to throw out the "Filler" file between uses of DiskFiller.