If you asked me to name just one program I don't think I could live without, I would name a program I never actually remember I am running from one day to the next. It runs so invisibly that the only time I ever remember I'm running it is when I'm not running it. If that makes sense.
I'm talking about StrokeIt, the single most useful program I have on my PC. It's free for non-profit and individual use. Companies can buy a license for $10 - and it's well worth it.