Like several others, I recommend Darik's Boot and Nuke.  Even the most minimal algorithm (one pass of filling the drive with zeros) is enough to render the data on the drive unrecoverable.  Yes, I tried this out with TestDisk, which was able to recover data before the first pass with DBAN and unable to recover ANYTHING after one pass of filling the drive with zeros.

As long as the drives still work, I recommend DBAN.  I think it's a waste to destroy a WORKING hard drive.

If the drives don't work, then I recommend destroying the platters inside.  I recently had to destroy a hard drive.  I was able to find the screwdriver I needed for taking the drive apart at a Runnings store.  It was interesting to see what was inside the hard drive.  I took out the platter, scratched it up, and disposed of the hard drive remnants at a local household hazardous waste disposal site.

-- 
Jason Hsu <jhsu802701 at jasonhsu.com>