This is something many people and especially businesses worry about, or at least they should. Before throwing away a hard drive, returning a hard drive, or especially Warrantying/RMAing it, you should wipe the drive.
Linux provides the "shred" and "dd" utlities which work quite well. It seems even a single pass is good enough but by default shred will do 3 passes.
Here's an example of using shred in Linux (I use a custom made distribution from........