Oh man, I love this topic and I think about it all the time. Here are a few of my favorites, all of which are also free:
Everything is, as far as I’m concerned, the best way to find a file on a Windows machine. Searching for a file by name on Windows is an inexplicably awful experience, and Everything can pinpoint you to the exact location of what you’re looking for as fast as you can type the search.
VLC is a ridiculously useful piece of software for interacting with a video file. It can play just about any goddarn thing you can throw at it, and if that thing is kinda broken it’ll do its best to play you at least part of it. Absolutely staggering that it’s managed to be as good as it is considering how much money has been offered to its owner to sell it to someone to make it worse. VLC is also built on top of another useful tool,
FFMPEG isn’t just a juggernaut, it’s an army. If there’s a thing you want to happen to a video file then there’s an FFMPEG incantation to do it. It takes a little time to figure out how to speak its language, but if you’re fluent you can accomplish tasks that sound impossible in five seconds with a single command. Even a cursory knowledge of how to use it will save you dozens of headaches as a video editor as you massage input files into a format that’s usable.
For all Microsoft’s faults, it does maintain Visual Studio, which is just a good damn code editor. Does all the things you want it to, can be extended to be as much or as little of your pipeline as you want, will put on a tutu and do the hula if you ask it. It’s rare to find a tool that’s simultaneously so clean and so extensible.
And it probably goes without saying for anyone here, but 7Zip really is just the standard for compressing and decompressing files now (outside of a Linux machine anyway, but if you want to get into Linux utilities which are just good and do what they’re supposed to we’ll be here all day).