Microsoft has released their F2P flight sim title, Microsoft Flight.
Here is their pitch:
Experience the thrill and wonder of flight with Microsoft Flight. This free download gives you the opportunity to explore the skies over the beautiful Big Island of Hawaii in the state-of-the-art Icon A5 or the classic Boeing Stearman. Enjoy hours of gameplay with a variety of exciting missions, challenges that will test your skills, and hidden Aerocaches that will give you yet another reason to explore.
Games For Windows Live and a Windows Live ID is required to play. You can get started here.