A format will still leave most data behind, just because you can't see it, does not mean it is not there!
There are a couple of programs which will write 0's & 1's over an entire drive several times. It will overwrite pretty much all data on it and after 6 or 7 passes, most consumer recovery software will find absolutely nothing.
Programs like Ultrashredder, wipedisk and wipefile will do it for you.
A format will still leave most data behind, just because you can't see it, does not mean it is not there!
True, but it's surely good enough for most people? A full format? Of course, if you've got a load of Glitter-esque data to get rid of, then yeah it needs deleting properly.