Decent scrub ware.

[Agnostic Bear]

Just run a single 0-out pass, no data recovery company will accept it as they can't do anything to get the data back.

[Jahova]

DBAN, I have used it before to completely wipe my old PC's HDD. It took a long time, and I mean it, but it worked and seemed to be a good program.

[Recursion]

Just use DBAN or something to wipe the drive over a few times with 0s. Personally for my mum's lower school I never get rid of the drives, I always take them and find a use for them, that way only I can access them, they have data overwriting the old stuff and are formatted a number of times.

[Tomm]

I recommend you do it more than once. Using techniques such as Magnetic Force Microscopy (MFM) you can find out the previous values (possible to recover more than just the previous value as well) of bits on the hard drive, although this is really limited to Intelligence Agencies and hundreds of thousands pounds data recovery. So you may think you are safe just overwriting it once or twice directly on the blocks on the hard drive, but you are not. However, in reality I doubt someone would go to those lengths due to the VERY high costs.

[Agnostic Bear]

Just run a single 0-out pass, no data recovery company will accept it as they can't do anything to get the data back.

I will say this again, a single 0-out pass is all that is needed, everyone else giving all these "multiple pass oh god they will check deep" are idiots, a 0-out pass is all that is needed, no company will take a 0-out drive, it's simply not gonna happen no matter how much money you have or however many government agencies are snooping about after your precious illegal mp3s.