There is a free program here that is very small: http://dban.sourceforge.net/
But as someone already stated, you can use "dd" to overwrite the entire drive. Either:
dd if=/dev/zero of=/dev/hda
will fill the hard drive (hda) with zeros. Or:
dd if=/dev/urandom of=/dev/hda
will fill with random numbers. One pass will make it virtually impossible to recover anything off the drive. If you repeat the command, it will make it more securely erased. 3 passes will truely elimate any possibility of recovering anything.