Persistent key-value database with built-in net interface
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. 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.