PC Decrapifier, which helped me to remove a lot of pre-loaded and unwanted programs that were souring my fresh-out-the-box new-computer experience.
Malwarebytes, which found a trojan on a new computer I hadn't even used yet. In the blink of an eye it was gone.
Both programs are free for the basic version, easy to install, and don't, that I know, deposit any dregs on your hard drive. I've used Malwarebytes in addition to McAfee Antivirus and Webroot's Spy Sweeper for about 5 years and it always finds things these two don't.