Its airport specific, but if you go to here: http://www.passur.com/sites.htm it will give you a list of all the airports that offer the free service. The Boston one (http://www4.passur.com/bos.html) lets you replay the tracks from an archive, or view "live", though "live" is delayed by ?15? minutes in the name of security (and I think callsigns are blocked out on the live feed as well).