HTML5 will include a new WebSocket API. WebSockets facilitate continuous connections between a server and a client, so information can be sent back and forth without needing to reload the page.
This enables faster communication with the server for an individual, but also facilitates multi-user interactions on a site. Examples I've seen range from a simple chat to a gigantic collective Scrabble game.
Currently, the WebSockets API is fully supported in Safari and Chrome.