overwrite is a UNIX utility for secure deletion. It's based on Peter Gutmann's paper "secure deletion of data from magnetic and solid state memory".
Secure rm is designed to delete files in a secure fashion by overwriting files before actually deleting it
wipe is a secure file wiping utility. however, it does not set the media access bit on scsi commands, therefore it is not 100% secure unless your drive has no write cache. Read more