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.