For basic flight tracking, google works pretty well. Just put in a flight number (e.g., UA1234, NW321, etc.) and it'll usually give you a list of services with which to track.

Personal favorites:

http://flightaware.com/live/ (probably the most advanced)
http://www.fboweb.com/fb40/default.aspx (uses google maps to show location--use the "quick track" feature on the left side of the page)
http://tracker.flightview.com/amex/f...er/ft_main.asp (old fashioned basics)