Another is flightsim.com!
If you are into airliners then look at ifdg and Project Opensky there are amazing aircraft on these.
The list is endless really.
Remember that most aircraft built for FS9 will work in FSX but the panels dont! Yopu get these seperately mostly anyway so you can mix and match.
If you join a virtual airline they will have loads of aircraft you can download.
Good luck, have fun!