Free File / Disk Erasers

DBAN Boot and Nuke
Darik's Boot and Nuke (DBAN) is a self-contained boot floppy that securely wipes the hard disks of most computers. DBAN will automatically and completely delete the contents of any hard disk that it can detect, which makes it an appropriate utility for bulk or emergency data destruction.

SDelete v1.61
SDelete is a command line utility that takes a number of options. In any given use, it allows you to delete one or more files and/or directories, or to cleanse the free space on a logical disk. SDelete accepts wild card characters as part of the directory or file specifier. SDelete works on Windows 95, 98, NT 4.0 and Win2K.

Eraser is an advanced security tool (for Windows), which allows you to completely remove sensitive data from your hard drive by overwriting it several times with carefully selected patterns.

Secure Delete
This program allows you to specify a directory (and optionally all subdirectories) in which all files should be deleted. When each file is deleted the contents of the file are filled with random data prior to deleting the file.

Autoclave v0.3
Autoclace is supposed to securely erase all the files, all the free space, the partition table, the boot sector, etc. This includes the operating system and any preinstalled packages your hardware vendor may have included on the drive when you got it. When Autoclave is done running, you should have a hard drive filled with zeros, and nothing else.

Overwrite (Unix)
Overwrite is a UNIX utility that tries to make data recovering harder. What overwrite does is to overwrite files using random patterns and deterministic patterns.

Wipe (Linux)
Wipe is a secure file wiping utility.