Here is a list of some online squadron sites. Over a time I'll be adding to the following list (also editing and deleting as required). The page is referenced from the 'Flight Sims' links section on my sidebar. My favorite server is Norwegian Server, one of the more challenging servers you can get into via Hyperlobby (download the 'client' to get started).
RAF 662 (Big Group of Squads, 'Golf Squadron' is mainly Austalian)
Rogue Squadron (home of several servers and squad's - vibrant)
RACF - FB (comprehensive)
The Real Aussie Airforce
Air Group 51
No 76 RAAF Squadron
52 minutes ago