Weawow is a completly (also ad-)free weather forecast app run basically solo by a Japanese guy. I was surprised when I found this app that it was so good in every aspect that I had to donate the guy. It has has more than half a Mio. reviews on google play with an average of 4.9 . Idk of any free app with that many reviews having this kind of rating, well deserved.
Further honorable mentions:
- Vivaldi browser
- Joplin notes app
- nextcloud
- wikipedia (obviously)
- Γffi
- Signal
- keepass
- rif for reddit (R.I.P)