qBitController - Control qBitTorrent servers from your phone. UI is really great; I use it regularly to download some media on my server (an always on old laptop) when I’m at work.
Pano Scrobbler - Connects to Last.fm to scrobble any music I listen on my phone. Also nice UI.
Rush - Fetches and shows lyrics for songs you search or whatever is currently playing. Can handle synchronized lyrics and sync with your player.
Auxio - Simple music player. Of the few I’ve tested, it handles metadata the best, which makes it work very well with other music apps (Pano and Rush).
Keysh - Assign custom actions for the volume buttons. I use the default script which makes the buttons turn volume up/down on short presses, and skip forward/backward a song on long presses. I use it pretty much daily to control music without unlocking my phone (and not using the broken controls on my headphones).
MJ PDF - PDF reader, simple UI, very fast.
Material Files - File manager, nice UI and feature set.
Ruler (Privacy Friendly) - On-screen ruler. Comes in handy sometimes.
Binary Eye - QR and barcode scanner. Can preview and copy the text before handling it.
You should try the F-Droid version 2.0, it’s currently in alpha but it is completely stable. They have revamped the UI completely, switched to Material You and it feels much more modern than the original design. It has a few new features too!
Been using it for a few weeks and it is just so nice to use now.