According to this Wall Street Journal article, FlightStats.com had the most accurate and up-to-date information. For those of you with a web-enabled cell phone, they also have a mobile-friendly website here. I’ve used FlightStats in the past and found the information very accurate – they even provide the gate number of your flight if it’s available.
I recommend skipping the airline websites, as their updates are flaky at best, and see if FlightStats.com works for you.
Happy flying and may you (almost) always get home on time.