Basically God of God games/sandboxes.
A super-deep intricate in-depth program sold on a quantum computer, that immediately becomes in of itself a CantGo Tu Machine that can generate pretty much every perceivable thing to the mind, and more.
Though I figure if a bunch of people use it and collide within the realities 'stored', it may make some cyber/'fake' people scared half to death when their sky rips open and fish falls out of the tear.
Unless it does something to differentiate from what it generates, if it generates on a frequency, plane, or level of some sort.
Basically the God of God games. From only you know what to the 100th of a quark, and even smaller.
You could go from 'Big Bang' to whatever.
You could play The Sims: Super-Real Edition.
You could spend hours constructing mass into things as small as fibres and as large as planets, galaxies, super-clusters...
You could make a universe that operates on the Dickbutt spectrum.
You could run a simulation of events so similar to your life, and watch through your eyes, except for the fact that it's not you but a simulation.
You could make a universe that plays out to some TV shows/Movies. 'IRL' Terminator.
You could create a reality and watch it split, and watch those realities split, and watch them also branch off, and see every single possibility.
You could put one atom in nothingness and watch it not do anything at all.
or plop Sad Larry in a small grey room all by himself and constantly throw party hats at him.
or create life and care for it, then get mad at it, then feel bad for being mad at it etc
Infinite fractals of Unobtanium.
Or create a universe full of rocks.
Or just colour everything blue.
Hell you could be watching yourself watching yourself watching yourself. What an asshole, he's wearing a sweater.
But when does the simulation become a reality for someone or something else? What happens when you pull that plug?
What if the inhabitants develop some Sliders wormhole technology and slide into your reality/universe, and take it over? Kill their creator and manipulate their world without insight?
best game ever
Or maybe something like Singularity (http://en.wikipedia.org/wiki/Singularity_%28video_game%29), but with more depth and different choices/events/setting/outcome/etc because time travel is wack.