Redis - A persistent key-value database with built-in net interface written in ANSI-C for Posix systems

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

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

A twitter clone by Redis + PHP
http://code.google.com/p/redis/wiki/TwitterAlikeExample