http://www.joelonsoftware.com/items/2007/10/13.html

It's pretty rare for airlines to be honest about delays at departure time. Rather than update their online databases when they know they're running late, they wait until the flight has taken off, because then they only have to update it once. It's pretty maddening. I can't tell you how often I've been standing at a gate with a big LED sign that says ON TIME for, say, a 5 pm departure, when the current time is 5:30 and there's no aircraft in sight.

For frequent travellers this may give you enough information to get rebooked before everybody else does. For example if there's an 8AM flight and a 10AM flight, but the 8AM flight is running 3 hours late, there's going to be a stampede to get rebooked on the 10AM flight, which might only have a few empty seats. The first person to figure this out is going to get one of those seats. This app is very reasonably priced for the audience of frequent flyers who would benefit from it.