Check out airlinesimulation.com. They'e just released version 5.1, which has tons of extra features compared to version 5. In a couple of weeks, version 6 will come out. It will have all the same features as v5.1, and more! However, v6 will only run on Windows XP
, and 2000, whereas v5.1, will run on ME
, 98, etc.
You can lease aircraft, buy aircraft, schedule routes, configure the aircraft, buy, or lease hub facilities, contract catering companies, make alliances with car rental companies and hotels, and tons of other stuff!
Hope this helps!