One of the most accurate simulations available, Kerbal Space Program is now available on Steam for $22.99 USD.
The game is a meticulous space program management game in which players must design their own space exploration vehicles to launch into space, and either orbit the Earth or even land on the Mun (the game's version of the Moon). The game is accurate to the extreme that even launching into space is a challenge.
The difficulty is mollified by the Kerbals themselves, little green astronauts who pilot the spacecraft. Much of the fun derives from their happiness - or, more likely, outright terror - as the ship launches.
Check out the Steam launch trailer in the video below.