Aces High 2
 

Well, I just wanted to point out about Aces High 2. It's an MMO realistic WW2 flight sim. I have never seen less than 200 people on and have seen up to 500 people on at the same time. Since it is MMO it has a $15 monthly fee, but you can download it and get a two week free trial.

If your interested you get get it here. Fly Aces High.

Edit: I really like this game myself, and have been playing preatty much constantly since I downloaded it three days ago. You can also use tanks, ships and AA.