Most people know that simply deleting a file won't stop someone from downloading important data from your hard drive. Sure you could do a 35-pass Gutmann wipe or run a program such as Darik's Boot and Nuke, but if you really want to feel secure, you need to bring in the heavy artillery.
Maker Wiebetech claims that its Drive eRazer Ultra will make your data disappear in a fashion that's good enough even for the Department of Defense (whatever that means), or you can use any of ten other modes if you think the DoD specs aren't good enough.
Unlike the software-based solutions mentioned earlier, the Drive eRazer Ultra is a physical box that you connect directly to your SATA or IDE drive. Wiping runs at around seven gigabytes-per-minute, so it should make pretty quick work of your drive. Best of all, after it is completely erased, the drive can be reused just as if it were new.
The cost of all this security? Just $274, or a lot more than the hard drive probably cost new. What the heck were you hiding on there anyway?