What are your favourite FOSS mobile apps?

I’m looking for alternatives for everything, from messaging to ebook readers to RSS readers to podcast players and so on.

Particularly interested in music players that can handle m4a files from when I was buying music from Apple not realising how blockheaded that was.

Hopefully apps that care about user privacy.

I’m on Android, but feel free to share iOS options for anyone who might be helped by those tips, too.

Can I get a boost, @opensource?