FlightStats is a travel tool that provides a wealth of flight-related information based on real-time data and past statistics. On the main page, you can check on scheduled departure and landing times, as well as actual take-off and projected landing. You can also elect to receive an e-mail or SMS alerts if the flight status changes, which is highly useful if your carrier or Web-ticketing service doesn't offer such a feature. Plus, there's a map view that provides at-a-glance on-time info for airports across the country.
In addition to gathering information by flight number and date, you can check on data by airport or by route for a limited array of dates and times. There are also a variety of other tools on the page, such as widgets, travel deals, and airport information.
As any good on-the-go travel tool should, FlightStats has a very good mobile-friendly site and applications for certain handsets. It's the one you'll use when you're hanging around in the airport wondering just how far away a flight you're waiting for is.