The theory of information and computing seems pretty fundamental, and not necessarily tied to what we typically think of as a computer, with CPUs, RAM, SSDs, etc. In a way, a card catalog full of index cards and run by a bunch of people is a computer too. Maybe this isn't an incredible revelation, but it's still interesting to think about.
Another interesting and radically less complex example that I have personally found useful is the Pile of Index Cards system. 
I have heard that Vladimir Nabokov used to write his works on index cards. The Wikipedia article on him mentions this.