Redis - advanced key-value store
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.
It is possible to think at Redis as a data structures server, it is not just another key-value DB. Redis supports operations like atomic push and pop of elements on lists, taking ranges of elements from this lists at once, trimming of lists, server-side intersections of sets and even sorting data.
Redis has client libraries in all languages. To prove the ability of Redis, Twitterclone project has been created using PHP and Redis.