In my most humble opinion, Thunderbird is by far the best email client you can get for your money (or no-money) because it’s free and makes email work.
LilyPond – music typesetting through code. It’s not for everyone but many non-coders may like it. It’s free to use, so why not give it a trail run?
An open source, multi-faceted eBook manager. You may not actually need one of these but if you like the idea, it’s free to try and free to keep.
There is a hidden cost to vendor lock-in that is most easily illustrated by a very common household item. Razors and blades, printers and ink . . .
Create stunning computer generated graphics with only the investment of your time and effort using the open source vector graphics software, Inkscape.
While free is a big word in use these days, some things are more free than others. Open source and Libre in software grant complete freedom.