KDE Filelight - Filelight is an application to visualize the disk usage on your computer
KDE Okular - Okular is a universal document viewer developed by KDE. Okular works on multiple platforms, including but not limited to Linux, Windows, Mac OS X, *BSD, etc.
KDE Connect - KDE Connect provides various features to integrate your phone and your computer. It allows you to send files to the other device, control its media playback, send remote input, view its notifications and may things more. It is available for (mobile) Linux, Android, FreeBSD, Windows and macOS.
All of these are Under GPL-2.0+ license