There are no true expansion packs for FS9. Instead, individual developers create addons for FS
. Probably the most well known add-on at the moment would be the PMDG 737 series, which combines all of the default flightsim features and more, such as a working FMC. Their website is precisionmanuels.com. Just be warned, its hard on a system.
Some other good addons are:
Aerosoft German Airports
Simwings Canary Islands
Simflyers scenery line (hard on frames)
FSPlanet Photomesh series
Holy sh*ts and burritos.