On Steam a few weeks ago when it was on sale for the first time there I believe.
I am 29 years old now, so you can understand that very few games/programs can impress me. I kind of feel I have seen almost everything, from text adventures to open worlds so many times, that very few things can keep my interest.
So call me a weirdo, I probably am, but innovation is what I seek the past 2 years and that is the reason I bought instantly Universe Sandbox, Achron and Rock of Ages.