The following database systems are now being categorised as NewSQL, this basically means they are traditional SQL, ACID compliant databases, but are built with the underlying philosophies that allow NoSQL databases to scale horizontally.

MariaDB Cluster

MariaDB Cluster, this is basically MariaDB with the Galera Cluster Manager. A good writeup of a conversion from MySQL+MMM is available here.

Translattice Elastic Database

Translattice, the TransLattice Elastic Database (TED) is a Relational Database Management System that provides ANSI-SQL support, the ACID transactions enterprise applications require, and the ability to scale-out across wide distances using ordinary Internet connections.

Translattice is based on PostgreSQL 9.x, with the storage engine replaced by Translattice’s distributed storage engine.

NuoDB

NuoDB, NuoDB provides the ability to replicate data globally in real-time, which is not available with any other product offering.

Clustrix

Clustrix, With ClustrixDB, you simply add nodes to linearly scale your database. Clustrix provides the scale-out of NoSQL without giving up SQL and transactions. Drop-in MySQL replacement.

VoltDB

VoltDB, more than just the world’s fastest OLTP database. It’s a way of thinking about application design that shatters conventional wisdom about how fast you can accept data, decision and analyze it, and how far and seamlessly you can scale

InfiniSQL

New kid on the block InfiniSQL, claims over 500,000 complex transactions per second. Over 100,000 connections. On just 12 little servers. It compares itself to MySQL Cluster, and is probably very worth looking at. It was started by an ex Visa engineer.