Recently I was looking for highly scalable database management systems (other than RDBMS) and the following ones caught my attention.
Object database management systems:
ODBMS products like Cache claim that it can run SQL 5 times faster than relational databases. They also enable extraordinary transaction processing speed, massive scalability, and real-time queries against transactional data with minimal maintenance and hardware requirements.Have a look at Cache ODBMS @http://www.intersystems.com/cache/index.html
There are also other good ODBMS products like Gemstone, Cerebrum, db4o of which db4o is an open source, fully functional ODBMS with nice documentation. Have a look at it @ http://www.db4o.com
NoSQL database management systems:
These databases are referred as structured database management systems, which scale horizontally and don’t need fixed table schemas. They can service heavy read/write workloads when compared to traditional RDBMS. Some of the popular NoSQL database management systems include Google’s BigTable, Amazon’s Dynamo, and Apache Cassandra.