A Tic Tac Toe Chrome extension.
I made this mostly just because I thought it'd be a fun, easy way to learn how to create chrome extensions.
It is currently published on the chrome store: https://chrome.google.com/webstore/detail/tac/kkcofijmkjoenpklbaliedjkidcndnon
As of 2/14/18 there were 950 weekly active users.
If you'd like to develop it yourself, see the "Load the extension" part of the guide below. It literally takes 10 seconds to run a test instance. https://developer.chrome.com/extensions/getstarted