There are a fair few options if you know where to look. Are you looking for a realistic simulation, arcade action, or something in between?
I get a lot of enjoyment out of "lite" sims, such as Thirdwire's Strike Fighters 2 series, for me this involves a degree of reality along with being fun to play. There is also a huge modding community (based mainly at CombatAce.com), and there are quite literally thousands of user mods , including everything from air and ground units, maps to new campaigns.
Lomac and FC2 are a little more involved, but are also very enjoyable.