IL2 is a great game, and as seen is highly recommended. I recommend it as well. It can be modded to your liking, and can fly online.
Me personally I spend a majority of my time in Aces High. Its a online game with multiple arena's. you have a early war, mid war, 2 late war arenas. A axis vs allies arena, Dueling arena, Training arena, and special events arenas. You can down load the game for free, and fly off line for free all you want. You also get a free 2 week trial to play online, and after that you have to pay to play in the arenas. Comming soon is a WW1 arena. The neat thing is they are going to start testing a new damage model with the WW1 planes before it is implemented into the WW2 birds.