FlightGear has their FlightGear Flight Sim V3.0 for Windows, Mac, or Linux as a download for FREE for a limited time.

FlightGear is an open-source flight simulator. This new version contains many exciting new features, enhancements and bug fixes; include integration of the FGCom voice communications client within the simulator, improved terrain rendering, faster scenery loading, and improved usability. This release also coincides with the release of FlightGear World Scenery 2.0 – massively improved scenery data covering the entirety of the planet and incorporating OpenStreetMap roads and detailed terrain information from a variety of sources.