Graph database is a great tool for mapping and traversing data with relationships. But doesn’t RDBMS have relationships - one-to-one, many-to-one etc? Why should we go for graphs? Its simple! Graph databases are best suited for those cases in which a graph comes to our mind instead of a table! I finished the online course on Neo4J two days back. Its a really simple graph database, and a great starting point to experiment graph databases.
Now the NoSql databases I am familiar with are HBase(tried it about a year back), Redis and Neo4J.
Read and explore stuffs outside your domain. Happy coding :)