A simple plugin that outputs player names and the total count to a JSON file.The file is available in the root of your Minecraft server directory (stats.json).
What you do with this information is up to you. I personally use it for provide a front-end for a server status site.
My web front-end is Bootstrap based and can be viewed at http://128keaton.com/applecider/ The source for the plugin and a web-frontend is available on GitHub.