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.
If ever there was a case of too much choice, free fonts is it. Get started on thinking if you want to use them, why, and what to do about it.
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.