There is a lovely boot CD that will erase your HDD to the highest standard (per the National Institute of Standards and Technology) by issuing a "Secure Erase" command to the hard drive. If your HDD is modern (and this command is over five years old), it will erase itself over the course of the next few hours.
I use the Parted Magic boot cd, using the Erase tool and selecting the Secure Erase option (last on the list, I think)