NoSQL Databases: Why, what and when by Lorenzo Alberton.
When I posted RDBMS in the Social Networks Age I did not anticipate returning the very next day with another slide deck from Lorenzo. But, after viewing this slide deck, I just had to post it.
It is a very good overview of NoSQL databases and their underlying principles, with useful graphics as well (as opposed to the other kind).
I am going to have to study his graphic technique in hopes of applying it to the semantic issues that are at the core of topic maps.