X-Plane is a powerful flight simulator with a realistic flight models. Welcome to the world of props, jets, single and multi-engine airplanes, as well as gliders, helicopters and VTOLs such as the V-22 Osprey and AV8-B Harrier. X-Plane comes with about 40 aircraft spanning the aviation industry. Scenery is almost world-wide and you can land at any of over 18,000 airports, as well as test your mettle on aircraft carriers, helipads on building tops, frigates that pitch and roll in the waves, and oil rigs.
X-Plane also has detailed failure-modeling, with 35 systems that can be failed manually or randomly, when you least expect it. You can fail instruments, engines, flight controls, and landing gear at any moment. It is extremely customizable, allowing you to easily create textures, sounds, and instrument panels for your own airplanes that you design or the planes that come with the sim.
X-Plane minimum requirements include: a G4 or G5 1GHz +, 512MB RAM, OpenGL and 16MB VRAM, and a USB Joystick/Yoke.
More information and demo download link here.