Kerbal...

Kerbal Space Programme: What a fantastic way to:

1. Indulge in running your own space programme
2. Learning about orbital mechanics and vector calculus (yay!)
3. Using countless hours in learning how points 1 and 2 above work
4. Building space stations that make Mir look thought out :-)

 

Now you'll have to forgive me, I've been testing landers for an Eve mission all day and I've a launch window coming up...