Orbiter
Anyone who knows my PC habits can tell that I am a sucker for various simulation games, be it rail network simulation like "Sid Meier's Railroads!", city simulation like "Sim City", racing simulation like "F1 Challenge '99-'02", "rFactor", "NASCAR Racing 2003" or anything else that aims to simulate things as close to reality as possible. Given that I also happen to be very interested in space things, it's not difficult to predict what I will say next.
"Orbiter" is the best game in the world, period. It's free, it has awesome graphics, it's as realistic as possible (as long as you consider Newtonian mechanics realistic, but there aren't much objects, flying at near speed of light, so it's not a problem). What else would you want? There are many plug-ins for the Orbiter as well, allowing you to add multiple new spaceships, new objects of Solar system (only the main objects are represented in the basic game, and Pluto is not one of them) and even new fictional star systems.
Well, OK, "Orbiter" is not the best game in the world yet. It is kind of aimless, because you're given a ship and you can do absolutely anything you want. Well, not ACTUALLY anything, because you can't crash it anywhere - either it will pass through an object or simply bounce off. Hurray for ethereal space stations and rubber planets! Anyway, I still find it a very entertaining way to waste my time. The learning curve is kind of steep at first, because you have to learn a lot of names and signs ("periapsis altitude", anyone?), but the first time you enter the orbit is awesome. Honestly, I haven't yet achieved anything more, but I'm willing to try. The docking with ISS or travelling to Moon (and fictional Brighton Beach base in it) are great challenges, and travelling to Mars is, of course, even more so).
If only there were some scripted missions in the game and some collisions and explosions, that would be the Ultimate Computer Gaming Grail for me. Maybe some day they will hear my plead...
