It's fast, it gives you six degrees of freedom (6DOF) and it's open source too! Fly Dangerous just had it's second big upgrade since entering Early Access.
Flight Of Nova is a solo-developed space and atmospheric flight sim currently in Early Access, it looks quite impressive and it now has a Native Linux version.
X-Plane 12 from Laminar Research is a realistic and powerful flight simulator with real-world physics, accurate aircraft systems / behaviour and an immersive simulation of the world.
