Did some R&D for my brother and wanted to document the best options for securely erasing hard drives for sale or donation.

Disc Wipe Utility – Windows executable and source (should compile on UNIX/Linux systems).
http://sourceforge.net/projects/disc-wipe/

Eraser – Windows scrub utility
http://sourceforge.net/projects/eraser/

diskscrub – UNIX source code
https://code.google.com/p/diskscrub/