I was shocked today when I loaded their page up... I thought I surfed onto the wrong site for a moment.
The thing I don't like about FlightAware is how it tracks specific routes for specific flight numbers. For example, flight AIR123 might fly from Toronto the Chicago in the morning, and Chicago to LA in the afternoon. It will only show one of the two routes, no matter which one is active. Great site otherwise though!
...and I don't think you have to register for any updates, but is probably recommended.