Redis is an advanced key-value store. It is similar to memcached but the dataset is not volatile, and values can be strings, exactly like in memcached, but also lists, sets, and ordered sets. All this data types can be manipulated with atomic operations to push/pop elements, add/remove elements, perform server side union, intersection, difference between sets, and so forth. Redis supports different kind of sorting abilities.
Redis supports trivial to setup master-slave replication, with very fast non-blocking first synchronization, auto reconnection on net split, and so forth.
Redis is written in ANSI C and works in most POSIX systems like Linux, *BSD, Mac OS X, Solaris, and so on. Redis is free software released under the very liberal BSD license.