You should be looking for something along the lines of Active@Kill Disk, which is a low-level hard drive format and erasing tool. These tools not only delete data, but they also run sequences of random bits across the entire drive in several layers, ensuring that the original data is all but impossible to recover.

If you need to be sure the data is gone for good, look for an application which conforms to the US Department of Defence's data clearance standard.

Another good method is to melt or dissolve your hard drive, though that is a little less practical and may make it difficult for you to continue to use the drive. Does a good job of getting rid of data, though.