This shell script will fill your hard drive with random bits, and then zeros. It will do this seven times. It works on UNIX, Linux, and Mac. Make sure to change /dev/sda1 to your desired drive. For the extra paranoid, run truecrypt afterwards and burn the password. Cheers!

Nuking-hard-drives-the-UNIX-way.sh

#!/bin/bash
echo "Beginning wipe"
for i in {1..7}
do
  echo "Starting pass $i..."
  dd if=/dev/urandom of=/sda1 bs=1M
  dd if=/dev/zero of=/sda1 bs=1M
  echo "Pass $i complete."
done
echo "Wipe complete."