Flightsim.com for virtually every addon you can imagine.
Avsim.com is almost smilar to Flightsim.com but you do not have to register
CSi for MD-11s and B717-200s
POSKY for gmax B767-200s, gMax B767-300s, gMax B767-400s, B747-100s, B747-200s, B747-300s, B747-400s, B777-200s, B777-300s, gMax A330-200s, gMax A330-300s and soon gMax B757-200s and gMax B757-300s!
FFG for DC-10s, KC-10s, B757-200s, L1011s and soon gMax Saab 340s and gMax A340s
Euroflight for B737-900s ,B757-200s and F100s and Fokker G1s
Yeodesings for B767-200s, gMax B737-200s and soon DC-9s
Allied FS Group for A340-300s, B737-600s, B737-700s, B737-800s, gMax YS-11s
Project Airbus for A300-600s and the Begula
Historic Jetliners Group for DC-8s, B707s and KC-130s
FeelRealFlight for HIGHLY REALISTIC repaints of POSKY aircrafts
International Aircraft Design Group (IADG) A320s, Avro RJ-85s and ATR-72s
DazeFlight for lots of addon scenery (Some not on Flightsim.com)
Check this thread for more
DMK(TG773/772) PEN(SQ772/MH734) PER(QF333) PEK(CA763) HKT(MI320) HKG(SQ744) ICN (OZ333), CJU,TAE(OZ734) TPE(3K320) KUL(T