Quokka is a hobby UCI chess engine that I developed in C++ from scratch during the winter of 2014.
It plays at an Elo rating of approximately 1800.
You can play against Quokka right now, without having to download anything! Quokka has an automated bot account that is currently accepting challenges on lichess.org.
Simply go to https://lichess.org/@/QuokkaBot and send it a challenge. It will only play one person at a time, but will accept pending challenges after each game.