Expert Flyer is a pay site, though they offer free 5 day trials and free alerts when the seat you want on a flight opens up.
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.
They’ve now added a ton of Star Alliance airlines to their award search feature, which means you can get email alerts for when the space you want opens up.