Windows/Mac/Linux (All platforms): Darik's Boot and Nuke does what it sounds like, so it's not a tool you want to mess around with unless you really want everything securely wiped off your system. If you're donating or otherwise handing off your hard drive, however, it's a serious tool for erasing data so it's really, really hard to ever find again. You load Darik's tool onto a CD, DVD, USB flash drive or even a floppy disk, and after it boots, you can either choose which mounted hard drives it should wipe clean and in which fashion (with varying numbers of over-writing to meet the standards of, say, the US Department of Defence or the Royal Canadian Mounted Police), or use the "autonuke" option to wipe everything gone for good. It worked flawlessly on some non-partitioned hard drives I wanted to donate to a local charity. Darik's Boot and Nuke is a free download; owners of dual-core processors should head for the 2.0 beta.