FlightGear is a flight-simulator created by volunteers, and released as free, open-source GPL software, available for download here. FlightGear has a graphical user interface for flying, and different types of flight models available. It has been used for academic research, education, and for fun.
Since version 3.4, FlightGear provides a new experimental feature: volumetric grass. This feature renders grass on the terrain, for example, at airports. An experimental feature, it isn't enabled by default, but can be activated by adding a few lines of XML to the right place. See also the forum discussion.