Martin Fowler's Introduction to NoSQL
Martin Fowler presented this rapid introduction to NoSQL databases at GOTO Aarhus Conference back in 2012. Here's the abstract:
Martin gives a rapid introduction to NoSQL databases: where they came from, the nature of the data models they use, and the different way you have to think about consistency. From this he outlines what kinds of circumstances you should consider using them, why they will not make relational databases obsolete, and the important consequence of polyglot persistence.