For general application simply overwriting a sector is good enough. These 'data recovery' programs that are available for a few hundrew bucks are obviously only capable of retrieving data that still physically resides on the disk surface. So if it's just the MFT that's gone bad or the partition table for example, then these programs will work as the reference to the file may be gone, but the actual data is still there and can be found by searching for file headers etc. But when the sector containing the data is physically overwritten then the data is, for all practicle applications, gone.
There are numerous 'standards' for erasing and sanitising HDDs. People often refer to the DoD standard (American Department of Defence) as the one to use, but any method that overwrites the sectors will be a feasible method. Some methods simply write zeros, other write certain patterns of data numerous times etc.
One thing to consider though is that if these software programs are used to 'wipe your data' they only have access the disk through the program itself, through the OS, through the BIOS and the disk controller. There will be areas on the drive that are not accessed (and therefore not wiped) as they have been marked as bad (added to the G-list which is a growing list of bad sectors stored in the drives service area) and the data that is there will have been moved to a working sector so that it can be accessed, but the old original copy will still reside. These software programs generally will ignore these sectors. These sectors can still be read with the right tools, so the disk is not 100% 'wiped'.
To cut a long story short if you want to do a pretty decent job of erasing most of what's on your disk before you through it away, then overwriting the data that's on it (with one of these programs or which ever method you prefer) will suffice. If you want to erase absolutely 100% of every bit of data that is on there and ensure that no one will ever be able to recover anything at all, the disk platters themselves either have to be physically destroyed, or degaussed (demagnetised).
There is talk out there about magnetic force microscopy which reads the magnetic information on the disk bit by bit, so in theory a disk can be 'read' bit-by-bit, but from what I know this has never been successfully carried out and in terms of the data that would need to be gathered and reconstructed, it's not really a viable option.
I have R60k worth of data recovery equipment and an honours degree in digital forensics, I was just being silly
