FS Economy
 

This is pretty fun if you haven't tried it already:

http://www.fseconomy.com/

Its pretty much an online campaign for Flight Simulator 2004 or FSX that tracks your flights.

You can:
Purchase and own aircraft.
Sell aircraft you own to others.
Rent out aircraft you own.
Rent other people's aircraft.
Build/Construct FBO's anywhere in the FS world.
Rent out terminals.
Buy and Sell Supplies. (used to keep your FBO businesses running)
Buy and Sell Fuel. (used by People that fly in FS Economy)
Buy and Sell Construction Materials (used for constructing FBO's and Upgrades)
Take flight jobs (flying cargo, passengers, etc)
Issue flight jobs (create transfer jobs to offer others in FS Economy)

Cool thing about FS Economy is that its a persistent world. So, while you are logged off you can still make virtual money from the planes you rent out and facilities you own. FS Economy's website tracks all the movement of materials, aircraft, fuel supplies, and virtual job offers.

The only downside to FS Economy is it only tracks propeller aircraft and helicopters. So, you can't fly jets based on the way the software/website tracks and communicates data with the simulator.