See http://www.cubrid.org/cluster. CUBRID Cluster is an open source clustered DBMS for high scalability including global schema, distributed partition and load balancing features. It is a spin-off project from CUBRID project and supported by NHN.
Sequoia is a database clustering middleware offering load balancing and transparent failover. Databases are replicated over multiple nodes and Sequoia balances the queries between them. Sequoia supports online maintenance and recovery operations.