LibreOffice or other open source office suites. Rich word processors, spreadsheet, and slideshow software are seldom thought about but extremely important in the information age, and the duopoly of Microsoft and Google would like nothing more than to see the open source alternatives die so they can take full control of your documents.

Especially if you use Linux as a daily driver: KDE, GNOME, XFCE, Lxqt, other desktop environments. Unless you know how to do everything from the command line, they're probably the things allowing you to use Linux at all. Think about how much funding Windows or Mac development gets, that's because making a full graphical shell and suite of up to date system apps is difficult as fuck and they're massive codebases that require constant maintenance. One could even argue that their development and maintenance is a bigger undertaking than the Linux kernel itself yet most Linux users never think about them, nor do they have the backing of large companies like the kernel does because pretty much all of them use Windows on their workstations even if their server infrastructure runs Linux. And high quality graphical environments are absolutely critical if we're ever to have hope of Linux being adopted by the general public and not just developers and power users. If you use Linux as your main OS and have the cash to spare, considering tossing even a quarter of the cost of a Windows license you didn't buy to your DE of choice and do your part in ensuring that DE stays usable in the future.