[__jonas]

lichess is open source and has an android app

[Zizizizz]

For convenience...

https://lichess.org

https://play.google.com/store/apps/details?id=org.lichess.mo...

[teleforce]

Great, thanks for the suggestion.

Apparently, Lichess has 'Over the board' feature that allows you to play with your friend offline, nice!

[naet]

It also has a pretty good chess clock if you have a physical board but want to play timed.

[mat_epice]

It also lets you play an engine (stockfish) running on your phone. I play offline on planes all the time.

[jmfldn]

Written in Scala I believe which makes me love it even more (Scala fan here!).

[graphe]

Vouching for this, best chess app ever. Also offers a website!