Zeromizer

Free. Generates files populated with either all zeroes, all ones, or random bits. You could do all zeroes until you're nearly out of space, then all ones, to make your free space unrecoverable. It could also dimension a raw disk for Bochs or Virtualbox, or whatever arbitrary purpose you have.

Downloads for Windows XP/Vista/7/8/10/11:

Zeromizer 0.1 0.07MB