7-Zip
7-Zip is a free, open-source file archiver by Igor Pavlov for packing and unpacking archives. Version 26.03 is the current Windows release.
Windows
Open source
Open source: System & utilities
Open source programs in the “System & utilities” section: Archivers, Hardware monitoring, App stores for Android.
Archivers
Instead of WinRAR
Hardware monitoring
Instead of CPU-Z, SpeedFan, NZXT CAM
CrystalDiskInfo
CrystalDiskInfo is a free, open-source Windows utility that reads information from hard drives and SSDs and shows their health status.
Windows
Open source
App stores for Android
Instead of Google Play Store, APKPure, Aptoide, Huawei AppGallery
Aurora Store
Aurora Store is a free and open-source Android app that works as an alternative to the Google Play Store app, distributed under the GPL.
Android
Open source