Microsoft has just announced that their new Flight Simulator, which is due for release in Spring 2012, will be free to play.
There will be chargeable add-ons but the core game is free.
Please note this is a PC only game.
"Microsoft Flight will be available as a free download this spring, giving players the freedom to fly the skies over the beautiful Big Island of Hawaii, complete a variety of exciting missions, test their skills in flying challenges, or find hidden aerocaches on the island.
In Microsoft Flight, players view the world from above in a visually stunning and realistic representation of the earth, complete with region-specific weather patterns, foliage, terrain and landmarks. Players can choose to take the helm using highly rendered, accurate cockpits and authentic piloting procedures, or simply use their mouse and keyboard to control the plane in an exterior view. More experienced players can tailor the flight controls to match their skill level, making Microsoft Flight easy for beginners while still challenging for the most accomplished PC pilots."