HD memory wiper/shredder

[Kinnell <kinnell.t () gmail com>]

Hey all.

I recently had a friend ask me if I knew of any real good file
shredders/wipers.  He wants to be able to securely shred a file used
for an encryption key after it is no longer needed at that location. 
I couldn't think of any off of the top of my head but I figured this
list would be a great place to start.  So does anyone know of a good
shredder to use for this purpose?  Freeware would be preferred.

thanks,
Kinnell