What's fascinating to me is that index cards are a kind of spiritual predecessor to modern database systems and computers - perhaps even more closely related than counting devices like the abacus. Richard Feynman touches on this in one of his lectures [1] that's been linked many times on HN.

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.

[1] https://m.youtube.com/watch?v=EKWGGDXe5MA

For anyone interested in index card systems, the idiosyncratic method of Niklas Luhmann [0] is a fascinating example. Many additional interesting opinions on notetaking and modern database translations of classic note taking ideas maybe found on the website of the translator, Manfred Kuehn [1].

Another interesting and radically less complex example that I have personally found useful is the Pile of Index Cards system. [2]

  [0] http://luhmann.surge.sh/communicating-with-slip-boxes

  [1] http://takingnotenow.blogspot.com/2007/12/luhmanns-zettelkasten.html

  [2] http://pileofindexcards.org/wiki/index.php/Main_Page

The most impressive use of the index card is probably the Mundaneum: https://en.wikipedia.org/wiki/Mundaneum .

I am a fan of index cards, and only recently was thinking of buying a deck to replace my current GTD organizer.

I have heard that Vladimir Nabokov used to write his works on index cards. The Wikipedia article on him mentions this.

You might like the "Hipster PDA" then: https://en.wikipedia.org/wiki/Hipster_PDA