Voldemort
A distributed key-value storage system
...It automatically replicates data over multiple servers, and automatically partitions them as well so each server only contains a subset of the total data. It offers many other features such as pluggable serialization support, data item versioning and an SSD Optimized Read Write storage engine.
Voldemort is not a relational database or an object database. It is essentially a big, distributed, persistent, fault-tolerant hash table. This doesn’t make it ideal for all applications but does offer unique advantages, such as the ability to read and write scale horizontally, and the ability to combine in memory caching with the storage system so that a separate caching tier is not required.