OsmAnd~ is spectacularly good. If you spend a lot of time hiking, biking or just traveling outside, i also recommend Trail Sense (compass and a bunch of other things), Forecastie (weather, including wind speed) and BRouter (offline routing if you need a route but don't have signal).
QuickDic is also very useful if you are traveling to areas where you don't speak the language. Offline dictionaries with very quick lookup. It's worth installing AnkiDroid too, if you do spaced repetition for language learning.
I use KOReader for PDFs, but it's not great. Are FBReader or Librera any better?
Simon Tatham's Puzzles are a fun little distraction.
QuickDic is also very useful if you are traveling to areas where you don't speak the language. Offline dictionaries with very quick lookup. It's worth installing AnkiDroid too, if you do spaced repetition for language learning.
I use KOReader for PDFs, but it's not great. Are FBReader or Librera any better?
Simon Tatham's Puzzles are a fun little distraction.