ArityCalc is really neat, does graphing and a few other things

JRPN 16C is a take on the HP 16C , it’s both fun to mess around with and a great help when working with raw memory

μMath+ is great for working with complex problems

I use a NewPipe fork called PipePipe that is a bit more stable from my experience

Aurora Store is great

Markor is my go-to notes app, lets you write notes in markdown among other things

Trail Sense is neat both for its intended purpose as a general hiking toolkit, but also as a way to check phone sensors

Total Commander, because of course

Organic Maps is an offline maps app based on OpenStreetMap data, it’s great