Cyan Worlds (the makers of the breakthrough classic, Myst) are the absolute masters when it comes to creating (or re-creating) simulated worlds. Their skill with environmental subtlety and sense of place is rarely equaled.
Eventually, they created a full MMO...Myst Online, Uru Live.
This little-known online community is still operational and free of charge.
A fun retro game similar to Wolfenstein, except with orcs, goblins and endless fireballs. You find yourself in a maze-like keep where you have to find your way through while gathering treasure and staying alive.
The original game fit entirely on a single floppy disk and it nicely illustrates how an entire world can be built with the creative re-use of just a few pixels.
The Game of "Life"
"Life" is a primitive simulation model created in 1970 to explore the concept of "cellular automata."
Although it is old, it is an excellent illustration of how just a bit of code can flare out and generate complex building-block shapes.
Depending on what "rules" you set (by choosing pixels) Life will generate a unique set of fractal structures.