From LinuxReviews

There are many ways to erase your hard drive, rm can do it for you. But what if removing them is not enough, what if you need to make sure the data can not be recovered?

[edit] Using DD

dd is a handy tool used for making copys. It can also be used to quite securely wipe harddrives:

for (( i = 0;i<10;i++ )); do
 dd if=/dev/urandom of=/dev/hda && dd if=/dev/zero of=/dev/hda
done

This will overwrite your entire hard drive (in this case seen as hda by Linux) with random information ten times.

This tip will take literally days or even weeks to run on an average machine with a decent sized drive.

Huge thanks to John Cairns, Ritter and EnIgMa.

[edit] BC Wipe

The non-free (free trial) BC Wipe (manual page) from Jetico is excellent for securely removing single files. It also supports wiping the contents of directories recursively.