Tutorial: A Countdown App and a chrome extension crx; appspot.com;

Chrome Web Store Tutorial: A Countdown App:
http://londonwebstorenight.appspot.com/webstore/countdown/index.html

This is a packaged app as opposed to a hosted app.
visit chrome://extensions, make sure Developer mode is showing, and hit "Load unpacked extension". Select the "countdown" directory to add your app.

The Chrome extensions docs (http://code.google.com/chrome/extensions/overview.html)