EDIT The NoSQL Ecosystem by Adam Marcus (from the book The Architecture of open source applications): http://www.aosabook.org/en/nosql.html
general thoughts and comparison http://www.thoughtworks.com/articles/nosql-comparison
technical comparison http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis
a Master’s Thesis – Analysis and Classification of NoSQL Databases http://scholar.googleusercontent.com/scholar?q=cache:rQlbiz6bojAJ:scholar.google.com/+comparison+of+nosql&hl=en&as_sdt=0,5&as_vis=1