If you are concerned about identity theft, account numbers, etc. .....
http://driveslag.eecue.com/
Due to the recent MIT study concerning data recovery from old hard drives, we decided that the only fool proof means of data removal was complete destruction of the disk platters.
scanning tunneling microscope recovery techniques
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html
For this reason it is effectively impossible to sanitise storage locations by simple overwriting them, no matter how many overwrite passes are made or what data patterns are written. However by using the relatively simple methods presented in this paper the task of an attacker can be made significantly more difficult, if not prohibitively expensive.
In the time since this paper was published, some people have treated the 35-pass overwrite technique described in it more as a kind of voodoo incantation ...
However.....
with the ever-increasing data density on disk platters and a corresponding reduction in feature size and use of exotic techniques to record data on the medium, it's unlikely that anything can be recovered from any recent drive except perhaps one or two levels via basic error-cancelling techniques. In particular the the drives in use at the time that this paper was originally written have mostly fallen out of use, so the methods that applied specifically to the older, lower-density technology don't apply any more. Conversely, with modern high-density drives, even if you've got 10KB of sensitive data on a drive and can't erase it with 100% certainty, the chances of an adversary being able to find the erased traces of that 10KB in 80GB of other erased traces are close to zero.
