I would classify Hadoop as a "extreme record processing tool". Hadoop doesn't have transactional semantics which are offered by relational databases. It's really a batch-based system that allows you to work with large data volumes at scale.
HBase would be the canonical example of Hadoop integration with a NoSQL system. There's an article on Oracle's site about Hadoop integration with their NoSQL data store:
http://www.oracle.com/technetwork/articles/bigdata/nosql-hadoop-1654158.html