Notecards are essentially analog databases. I like the physicality of them – one can carry a concept and reference for that concept around in ones hand.

http://ryanholiday.net/the-notecard-system-the-key-for-remembering-organizing-and-using-everything-you-read/

Luhmann’s Zettelkasten takes this to a whole new level.

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