Google Chrome Background Apps is a feature that allows web apps to run without seeing them.
“Apps and extensions that use the new “background” feature can continue to run in the background—even if the user closes down all of Chrome’s windows. “Background apps” will continue to run until Chrome exits. The next time Chrome starts up, any background windows that were previously running will also be re-launched. These windows are not going to be visible but they will be able to perform tasks like checking for server-side changes and pre-emptively loading content into local storage.
One way you can use background windows is to preload content and data so that they are immediately available when the user opens your app. You could also issue HTML5 notifications to alert the user when important events occur—for example, a friend wants to initiate a chat session. There are plenty of possibilities here, and we look forward to seeing what you’ll do.”
More info: http://blog.chromium.org/2011/02/amping-up-chromes-background-feature.html
Sample app on the Chrome Web Store: https://chrome.google.com/webstore/detail/biecdmomngcfnkjelkbfjijebfednkno#