Simply formatting a drive has never been enough to get rid of data completely; otherwise, data recovery specialists would be out of a job. That's where Data Destroyer Disk Wipe ($14, free demo) comes in. Not only does it erase data on a drive, it writes and rewrites over that data with random bytes, making the wiped data totally unrecoverable.
Data Destroyer Disk Wipe works on any hard drive, memory stick, or floppy disk--anything that's easily rewritable and that is recognized by Windows as its own drive letter. Due to technical limitations inherent in the CD format (such as not being able to easily write to it without special software), it won't work on a CD-RW or DVD-RW.
DDDW worked just fine when I tested it on an SD memory card inside a USB memory card reader. That said, those used to fast formatting will be disconcerted that Data Destroyer Disk Wipe takes quite a long time to do its thing--an hour for a single Alternate Bit Patterns Plus Random Bytes pass. To meet the Department of Defense disk sanitization standards, you'd have to do three passes, which will take hours.
You have the option of excluding root directories and the files in them from the purge, but you can't get any more specific than that as far as exclusions. One missing feature that I'd like to see in later versions is the ability to recognize partitions. Data Destroyer Disk Wipe works only on specific drive letters, meaning drives that have already been partitioned; in order to wipe an entire drive, you have to run Data Destroyer Disk Wipe on one partition at a time.
Data Destroyer Disk Wipe also has the ability to wipe a swap file using a separate executable called wipeswap.exe. Swap files are used by Windows to temporarily store files during a RAM shortage, and sensitive information might be stored there. Wiping the swap file is useful because the swap file cannot be wiped on a restart in Windows 95, 98 and ME.
To wipe a swap file, you must reboot your system in DOS mode and run wipeswap from the command line, as it can't be wiped while Windows is running. For users of NT, 2000, XP, or Vista, the operating system can be set up to do this automatically, without the use of wipeswap.
If you're selling your PC, or donating it to charity, it's absolutely vital that you run a program such as Data Destroyer Disk Wipe on all your drives first, else you're setting yourself up for identity theft. Those who work with sensitive data regularly (or are reasonably paranoid) will also get a lot of mileage out of this.
Note: The demo version does not write to the disk except for purposes of estimating the disk write speed. Therefore it doesn't wipe disks, making it more or less useless except to see what the user interface looks like. The full version costs $14 for a single-user license.