The Mozilla Foundation is on a kick to show people just how amazing HTML5 can be, and to that end, they're releasing a series of free, open, ambitious in-browser apps to inspire developers and users. The latest of these is BrowserQuest, a multiplayer online role-playing game built completely out of native HTML, with no plugins, and with sourcecode for your learning, tweaking, and repurposing pleasure.
BrowserQuest can be played by thousands of simultaneous players, distributed across different instances of the in-game world. Click on the population counter at any time to know exactly how many total players are currently online.
Players can see and interact with each other by using an in-game chat system. They can also team up and fight enemies together.