Minecraft, actually. Specifically, the ReactorCraft mod by Reika. It basically adds
very realistic fission (pebble bed HTGR, normal U-235, and liquid-metal fast breeder reactors) and fusion (tokamak) reactors. Its an addon for his other mod, RotaryCraft, and is basically the end of the endgame,
especially the tokamak. It makes more power than even a large server could
ever feasibly use barring insane edge cases. Yes, its realistic in both technology required to setup and control the reactor as well as the
obscene level of output you can get from one. I got it built on my server, if you want an invite and link to the addon pack I'm using, lemme know. Always looking for new players!
[ETA]
I'm almost done. I can't believe it. The poloidal magnet (round thing in the centre) is operational, all magnetic containment pipes (the gold-lined pipes encircling the reactor) are charged to the point of insanity, the toroid magnets (the round things forming a ring around the poloidal magnet) are in place and full to bursting on coolant, the boilers and neutron absorbers are in place and ready to go, steam transport is established and two high-pressure turbines are on standby to begin operation, and the heating unit (the white structure on the right) is complete and awaiting the touch of the heat ray to begin producing fusion plasma.
I don't know what to do after this...this is the ultimate technological achievement. I could terraform entire dimensions with this power. I could level continents, drain oceans...run a farm that spans the entire planet,
anything. I have, at my disposal, power beyond the dreams of avarice. If this thing works after all this...I don't know what I'll do afterwards, but I will never have to worry about power ever again.