dban has an iso file that does multi pass scrubs. i use to have another free program, hold on. It was Eraser.
Dban is the best, probably hardest to use with its minimal interface, (you boot up the program, it doesnt run from windows) and you want to be careful using it. i.e. make sure you select the right drive you want to erase.
Oh also, File shredder. check that out