Let's talk about the #OpenSource software you're happy to *pay* for.
A one time "pay what you want" payment counts (such as for @thunderbird or @elementary). Recurring donations count too (Paypal/Patreon), and so do things like buying a "Pro" version.
MY ANSWER: Two terrific apps spring to mind which I've continued to use constantly.
1) Standard Notes
2) Aptik (#Linux backup software).
How about you?