Redis is a key-value database.  It is similar to memcached but the
dataset is not volatile, and values can be strings, exactly like in
memcached, but also lists and sets with atomic operations to push/pop
elements.

In order to be very fast but at the same time persistent the whole
dataset is taken in memory and from time to time and/or when a number of
changes to the dataset are performed it is written asynchronously on
disk.  You may lost the last few queries that is acceptable in many
applications but it is as fast as an in memory DB (Redis supports
non-blocking master-slave replication in order to solve this problem by
redundancy).

WWW: http://code.google.com/p/redis/