There are essentially two simulators that the majority of pilots use. Microsoft FSX and Xplane. They both have pretty much all of the airports in the world installed by default. The extras you install are the actual scenery, but the airports are there without any manual installation.
Sadly, sim software has been getting less and less user friendly as it becomes more of a niche product. Flight sims used to be a major gaming category in the early years of computers, but now with the gaming market having become so ruthlessly competitive, sim developers have fewer casual customers.
Nowadays, most sim enthusiasts are semi-pro pilots and aerospace engineers. The biggest draw of sims is being able to fly aircraft that you normally couldn't, like the A-10 which is masterfully simulated down to its tiniest details by DCS in DCS Warthog.