There are plenty of options out there, one I use is called DBAN (Darik's Boot And Nuke). It can be written to a CD-ROM or floppy disk. You boot off of it, and a few choices later you can be performing anything from a simple one pass zero-out wipe to a long 35 pass random type wipe. I personally go for the DOD-short method which, in 3 passes, writes random bytes, then the binary inverse of those bytes, then blanks the drive.

http://dban.sourceforge.net/

I never send a computer with a working drive to the scrap pile without running this (or BCWipe which does a similar thing).

Now if the drive is dead, I just take it apart. I then use the powerful servo magnets already in the drive and hold them up to various places on each platter, then take a hammer to them. Or... for the drives with glass platters, I simply shatter them ;-)

For those who enjoy the kinetic method - I had a co-worker from a prior job that took a couple drives out to a "shooting range" along with someone's .50 caliber rifle. Even at 100 yards, the drives didn't stand a chance. Shot through the top, the round made a nice hole clean through. Shooting through the side (read: thick) simply causes it to "explode" into many pieces.