Microsoft released a free trial version in 2006. It features two missions and a limited area (St. Maarten) for those who want to test the mechanics before buying.
Thousands of free aircraft and scenery mods exist. Microsoft Flight Simulator X Download Free Game
While FSX is not "freeware," there are legitimate ways to access it: Microsoft released a free trial version in 2006
You may see websites offering the full game for free. Please be cautious: Microsoft Flight Simulator X Download Free Game