Redis is an open source, advanced key-value store.  It is often referred
to as a data structure server since keys can contain strings, hashes,
lists, sets and sorted sets.

You can run atomic operations on these types, like appending to a string;
incrementing the value in a hash; pushing to a list; computing set
intersection, union and difference; or getting the member with highest
ranking in a sorted set.

In order to achieve its outstanding performance, Redis works with an
in-memory dataset.  Depending on your use case, you can persist it either
by dumping the dataset to disk every once in a while, or by appending each
command to a log.

Redis also supports trivial-to-setup master-slave replication, with very
fast non-blocking first synchronization, auto-reconnection on net split
and so forth.

WWW: http://redis.io/