If you are that worried it’s best to remove the drive. This way you are sure they cannot recover anything. For reviews of 5 popular hard disk scrubbing applications check out this link. One even follow the DoD’s policy for data removal.
Just a side note…Deleting all your data us not sufficient. You really need a program like those in the link. There are utilities that let you recover deleted files provided not much else has been done on the drive. That is why these programs read/write many times over the same sectors to thoroughly remove any trace of your data.