Ever play "AstroEmpires"? AstroEmpires is a browser-based video game where players gather into large groups called "guilds". The more successful guilds usually have databases cataloging explored galaxies, planets and moons. I invented a "new" method of getting this information to a database hosted on the web so that everyone could share information in near-realtime.
I'll be taking it down soon but I thought I'd describe its implementation here.
GreaseMonkey is a browser plugin that allows you to run scripts on top of any webpage you like after it has been loaded into the browser.
If the current galaxy being viewed was "unknown", a URI would be created with parameters and then a new "script" element would be added to the page with the "src" element pointing to my server.
Like so: src="http://mywebserver/script.js?update=true§or=55:43:55:11..."
I expanded the application to create reports of the "explored universe" on the same server that users could login to, and added "username" and "password" parameters so that only those authorized could use the site and the same authorization could be revoked if I so desired.
I eventually turned over most of the administration to a close friend and allowed him to change the database to Oracle. I allowed him to login to my server via RDP.
I considered writing an AJAX realtime chatroom application as well, but discovered this would be a significant challenge for a "spare-time" project related to a browser-based video game. :)