NoSQL refers to a class of databases (and data stores) that either don’t require a formal schema or are distinctly different from relational databases.
Popular databases that are considered part of the NoSQL generation:
Also see the Key-value Store topic.
A closer look at Redis — November 8, 2011
Getting to know the Redis key-value store
Seven databases in seven weeks — December 1, 2011
Take some time to explore seven different databases
PostgreSQL as a key-value store — December 13, 2011
If you're tempted by one of the key-value store databases that are super popular maybe you should take a look at PostgreSQL's hstore