It's pretty simple to make a digital clock in JavaScript. Once you learn about the date object and setInterval, you're home free.
* Show times for more than one time zone. (Defaults to user's time zone right now.)
* Add AM/PM (make it a 12-hour clock instead of 24-hour...or better yet, make a button to flip display).
* Alarm clock?
* Add the date (day, month, year).
* STYLE IT, OBVS.
* Use geolocation to pull in more information.
* An ANALOG clock, mwahahahahah!
* Change the font for the time display: a monospace font will look more like a digital clock, and wouldn't jar your eye as the variable-width string is recentered every second.
* Tie this to the US debt clock.