Flightstats.com will tell you the time is left the gate and the time it left the runway. It tells you the same for arrival, and if you want to get really detailed, it even tells you all sorts of technical details, like runway schedule updates.

You can also see the overall on time rating for that flight.

For the details, the site does require registration, but it is free. I relied heavily on the historical flight data on the site when I started entereing my flightmemory data.