Wow, that's an open-ended question! A lot of tools can be scary until you get used to using them. I remember a time when I thought a circular saw was a scary tool. (The fact that I tried to cut my foot off with one when I was about 12 might be a factor there. Cut through my shoe... and my sock... but somehow did not even scratch my foot!) More recently, a lot of metal-fabricating tools can be scary. Welding can be scary, even more so using a plasma cutter. And an angle grinder is certainly a tool to be respected! In the kitchen, I bought myself a proper chef's knife for the first time in my life a couple years ago. Very quickly learned that an extremely sharp knife can be scary! Back to the garage, air tools are just plain cool. They can make any job quicker and easier.
I guess I'll go back to the angle grinder. It's cool, interesting, extremely useful, AND scary. I bought my first angle grinder shortly after I started playing with welding maybe 10 years ago. It's one of those tools that made me say "how have I gotten this far in my life without owning this tool???" It cuts through anything. Most cuts I'd have fumbled through with a hacksaw or tried to do with an itty-bitty Dremel cut-off wheel, are child's play with the angle grinder! Cutting metal (or most anything else). Cleaning up sloppy welds. Cutting the head off of a rusted bolt. You name it.
There really aren't a lot of tools on that "how did I get by so long without owning this" list. Two that come to mind: a 12" long 3/8 drive extension is an extremely handy tool when you need it. And a 24" long breaker bar is indispensable for loosening most overly tight fasteners.
But, since you said computer software... Norton Utilities was always pretty scary! So much power to completely ruin your hard drive and data with the click of a button!
What a fun question!
What is the oldest hand or power tool that you own? (I still own and use the Black & Decker circular saw, Craftsman claw hammer, and Craftsman measuring tape that I bought when I bought my first house in 1989. Though I do own a pair of my Mom's old upholstery scissors that surely date back to the mid-70's.)