An RC flight simulator is a computer program that allows pilots of radio-controlled airplanes to practice on a computer without the risk and expense of damaging a real model. Besides the obvious benefits of its use as a training tool, simulators are also used for practicing new aerobatics and evaluating a model before buying it, and of course for flying when it's dark and/or raining.
There are a number of commercial packages available, such as Hangar 9's FS One, SVK Systems ClearView, Great Planes Realflight and IPACS AeroFly. There are also some freeware packages, the most popular being Flying Model Simulator (FMS).
Most simulators allow the importing of add-ons such as new models and landscapes. There are many websites dedicated to offering free content like this.