Seat Availability On Completed Flights
 

Is there a web site that will show you if a flight departed with available/ vacant seats? In particular if those seats were award or revenue seats.

Not that I know of.. but would be useful information.

You can see it on United, kind of. If it's the same day, you can do a Standard award search by schedule for the flights you are interested in. Assuming you have Expert Mode enabled in your profile, you can click on the Flight Details for the flight, and it'll show what availability was there when the flight left.