Kerbal Space Program

Kerbal Space Program (KSP) is a physics-based space flight simulator where you build your own rockets and space planes. You manage a space program for the 'Kerbals,' a race of green aliens. The game is famous for its realistic orbital mechanics and the 'trial and error' nature of rocket science. It is both incredibly fun and educational, teaching players the basics of rocketry and space flight.

While the sequel had a difficult launch, the original KSP remains a beloved classic with a massive modding community that keeps it relevant and highly playable today.