How to programmatically really clean Delete files?

So you are about to pass your work-computer to some of your colleague. How do you make sure you really delete all your personal data?

Re-formatting, Re-installing OS will not really solve the problem.

I searched around and found some programs does "Wipe out" disks.

This caught me thinking how does those programs work? I mean, What algorithms they use and how low level those implementations go?

Any ideas?