Here's a thread for you all to share screenshots of your space-efficient or interesting factory layouts. You probably shouldn't look at these screenshots if you think finding great layouts by yourself is a fun challenge, or if you don't (yet) care about getting the best possible use of space out of your factory, because you might just become infected with a bad case of perfectionism from reading this thread.

I'll start with one of my own designs:

How to fit 96 steel furnaces into a Mk1 building
Just in case you thought it couldn't actually be done, here's a steel furnace layout that smelts two full red belts worth of ore. It took me some hours of experimenting to fit all those things in there.

Just part of my current upgrade:
10 000 waffer/minute
http://i.imgur.com/zytEdbJ.png
Bob+angel+compressor

I'm playing a scenario where I'm trapped in a Mk3 factory building and need to bust through the ceiling using a rocket. There are unlimited resources coming from outside (except power), so I'm only constrained by interior space.
Here's my factory floor right before I research Recursion 1. I'd say this could be made a bit more challenging in a Mk2 building.

I'm playing a scenario where I'm trapped in a Mk3 factory building and need to bust through the ceiling using a rocket. There are unlimited resources coming from outside (except power), so I'm only constrained by interior space.
Here's my factory floor right before I research Recursion 1. I'd say this could be made a bit more challenging in a Mk2 building.

Why is your water not coming in from the outside?

Water just doesn't fit through a limited number of pipes in the amounts that will eventually be necessary, so I just enabled Creative Mode's infinite water recipe instead.

For these purposes I prefer to use Waterfill mod. https://mods.factorio.com/mods/Riley19280/Waterfill
It is very convenient at any factory level.