Make your data completely unrecoverable ...

If you’re ready to invest in a new computer, and your thinking about donating your old computer to a charity, local group or school, it’s important to make sure your computer’s hard drive is ...

I use http://www.heidi.ie/node/6 for free space wipe within windows. If not running within windows I'd use a linux live cd, although I don't think this meets your gpl ...