Free Flight Simulator

Flightgear is a free flight simulator you can download here:

http://www.flightgear.org/download/

There are quite a few aircraft to download here:

http://www.flightgear.org/download/aircraft-v2-6/

And here:

http://flightgear.bplaced.de/aircraftlist/

Liveries:

http://wiki.flightgear.org/FlightGear_hangars

Flight Gear is a very good sim. I've used it quite a bit, and it is very realistic. An interesting note on it is that another company took Flightgear changed the name and is selling it. They made no alterations to it and got it FAA approved which is a tribute developers of Flight Gear.