FSEconomy, or FSE for short, is a free flight simulation add-on that, as the name suggests, brings the elements of an economy to your flight simulation experience. Get paid for your flights, rent or buy aircraft, rent or lease airport facilities and more, all while interacting with an online community operating in the same simulated Game World as you.
 
While no longer an active pilot, I have been a member of FSEconomy since 2010 and continue to offer some add-ons in partnership with other players for the FSE Game World, which is almost an entity in itself beyond your simulator.