I've used something like this before:
http://eraser.heidi.ie/
You can have it overwrite the drive bit by bit in patterns, the more times the harder it is to recover things (and would require take-the-platters-out-and-read-the-magnets level recovery).
Though pulling out the platters and smashing them is really easy and kind of fun if you've never disassembled a hard drive before.