Algo Deck: an open-source collection of 200 cards on algorithms 6 years ago
If you're coding in a modern dynamic language
I see. I don't do that very much.
As for trees, your code is a tree. Every time you're working with a nested data structure, you're working with a tree. Trees are fundamental to programming and data representation in a way hardly anything else is.
I haven't really needed to write a program to traverse my code as a tree, but I guess you're right that I must have used trees in C at some point.