Key-Value Storages

memcached - distributed memory object caching system ( http://memcached.org/ )
memcachedb - distributed key-value storage ( http://memcachedb.org/ )
redis - A persistent key-value database with built-in net interface written in ANSI-C for Posix systems ( http://code.google.com/p/redis/ )
MongoDB - scalable, high-performance, open source, schema-free, document-oriented database. Written in C++ ( http://www.mongodb.org )
CouchDB - doc. oriented, written in Erlang ( http://couchdb.apache.org/ )

Riak - NoSQL, decentralized ( http://riak.basho.com/ )

RINGO - distributed key-value storage for immutable data, written in Erlang ( http://github.com/tuulos/ringo )

Dynamo - Amazon’s Highly Available Key-value Store ( http://www.allthingsdistributed.com/2007/10/amazons_dynamo.html )

Cassandra - highly scalable, eventually consistent, distributed, structured key-value store. "Dynamo 2.0" ( http://incubator.apache.org/cassandra/ )

Scalaris - scalable, transactional, distributed key-value store ( http://code.google.com/p/scalaris/ )

P.S. Storages overview from Richard Jones http://www.metabrew.com/article/anti-rdbms-a-list-of-distributed-key-value-stores