Expert Flyer is a pay site, though they offer free 5 day trials.
I find it invaluable. Historically it has been a must for any American AAdvantage member since it’s been one of the only sources for real-time availability of American Airlines flight inventory and because it also displays confirmable upgrade availability for American’s flights.
Two weeks ago they added support for awards across the Star Alliance. Now they’ve just added British Airways as well.