X-Plane (pronounced 'evil-day) is an aircraft simulator software that was developed and released by Laminar research as an add-on to the FSX game. It's one of the most popular simulators around and has been available for a while. Commercial versions are available for Linux, Windows, Mac OSX, and even iOS. It's not free, but is actually very cheap considering that it's very good. In fact, you can buy all the versions of X-Plane for the FSX game at a fraction of the regular cost!
This is because you can download more plane packs for X-Plane than other sims available. You'll find many scenery packs as well as many new and upgraded engines. You will also get a whole host of add-on features, including weather models, GPS terrain mapping, landing gears, landing gear animations, custom wings and tails, radio control aircraft, custom airports, airspeed indicator and much more. You'll also get support for multiplayer modes, meaning that you can fly with up to 8 people at the same time. This makes it ideal for multi-pilot groups. And if your group isn't big enough, you can always join a larger online game and split up the flying chores.
As mentioned, X-Plane has a huge amount of aircraft, so you may want to consider buying an upgrade to make your plane collection complete. There's no limit to what you can do with X-Plane, but the only limit is your imagination. It's great fun!