What app is so useful, you can’t believe it’s free?

It depends on how you define "app" and "free". But for free (as in beer) smartphone apps I really like.

• tidy
• Librera Reader
• these pre-packaged apks for the onnx text to speech engine which uses the piper voices that can be used in conjunction with the above Librera Reader to turn any compatible text doc to an audio book.