A new wave of databases categorized as NoSQL (Not Only SQL or No SQL depending on which interpretation you prefer) have burst on to the scene and offer some features not provided by relational databases that many applications have found invaluable and essential. Such features include document-oriented storage, the schema-less nature of documents, replication, high availability, sharding, map/reduce, and so on. Each of these bring something new to the table and allow applications to relay information and offer certain features they haven't been able to do with relational databases. Popular NoSQL databases include MongoDB, Couchbase, Redis, Apache Cassandra, Riak, and so on.
As such, it is not uncommon to encounter applications today entirely or partly backed by NoSQL databases. In fact, several of the applications we have used during the course of this book are backed up by NoSQL databases.