Barcode Scanner
My favorite barcode scanner, because of the UI, an optional history feature that makes sense, a good barcode generator, extra info about barcode types and the option to (automatically or manually) fetch product info via Open Food/Beauty/Pet Facts and MusicBrainz. F-Droid, GitLab
MixPlorer
Still the most feature rich and customizable file manager available, especially when it comes to networking and cloud support. Unfortunately not open source. Free via XDA (direct download via official Google Drive), or support the author via MiXplorer Silver (Play Store).
Smart Task Launcher
I'll probably never find a proper open source quick launcher like this: appoint actions to gestures in a hotspot region of the screen and open a launcher over other apps to quickly reach other apps, shortcuts or actions. Play Store
Tranquil Stopwatch
A new and reliable stopwatch for timing very long sessions. Doesn't use any service and has a nice OLED-friendly always on display. Really simple and just customizable enough. F-Droid, GitHub