SDelete

So you're leaving your job or passing on your computer to a friend and you want to make sure the files you've deleted are truly inaccessible, non-recoverable, gone forever. SDelete to the rescue. I could try explaining how this great command line utility works but its author, Mark Russinovich of Sysinternals, does it very well here.

SDelete is also a great way to clear up free space. I was running low on disk space on C and after uninstalling a couple unnecessary programs and defragging, I ran SDelete with the -z parameter. It cleared up an additional 233 MBs of space.

Notes:
- Free space may decrease temporarily while running the utility. At one point, towards the end of the program, I received warnings about low disk space that I ignored.
- As is always the case when running programs that delete files, use caution. Once SDelete deletes them, there's no recovering them.

I've also used Sysinternals Process Explorer and Whois with great success.