Zero free space on a partition?

I am looking for some method of writing 0 to the free space on a partition leaving all files intact. The partition is /dev/hda3 and its mounted /home. Back in the days of pgp there was a command ... something like ... pgp -w SIZE=XXXXXXXX dump where XXXXXXXX was a kb size greater then the remaining free space on the partition. Running this in any directory on the partion would fail when the size of the free space was reached and leave a file dump the size of all the previous free space. One could then rm dump and have cleaned all free space from potential prying eyes. I have yet to find the equivalent in gpg. Is there one? Is there some other trick to doing this? Thanks john