Postgres-XC is a write-scalable clustered database solution that is 100% free and open source. Postgres-XC offers a transparent connectivity model for your applications enabling applications to connect to any one of multiple nodes and provide the same consistent view of the data, with near linear scalability in a true shared nothing architecture. To applications, it just appears as one unified database, while hiding the complexity of the cluster from the applications.
Postgres-XC is targeted towards high-volume write scalability, and not data warehousing or reporting type of workloads.
More information about the open source project can be found at postgres-xc.