zerofree binary package in Ubuntu Precise i386

 Zerofree finds the unallocated, non-zeroed blocks in an ext2 or ext3
 file-system and fills them with zeroes. This is useful if the device
 on which this file-system resides is a disk image. In this case,
 depending on the type of disk image, a secondary utility may be able
 to reduce the size of the disk image after zerofree has been
 run. Zerofree requires the file-system to be unmounted or mounted
 read-only.
 .
 The usual way to achieve the same result (zeroing the unused
 blocks) is to run "dd" do create a file full of zeroes that takes up
 the entire free space on the drive, and then delete this file. This
 has many disadvantages, which zerofree alleviates:
  * it is slow;
  * it makes the disk image (temporarily) grow to its maximal extent;
  * it (temporarily) uses all free space on the disk, so other
    concurrent write actions may fail.
 .
 Zerofree has been written to be run from GNU/Linux systems installed
 as guest OSes inside a virtual machine. If this is not your case, you
 almost certainly don't need this package.

Source package