The blog post I referred people to the single most times in 2013 was this one:
http://madeofmetaphors.com/shapes
It's an amazing piece in that I've never seen the concept covered elsewhere, and it instantly filled a bunch of gaps in my knowledge and tied them together.
Brian's whole site and writing is excellent; that post, in particular, is a must read if you collaborate with other people at all. Highly recommended.