Key-Value Databases: Voldmort, Redis, Scalaris
Columnar Databases: Hbase, Cassandra, Hypertable
Document Databases: MongoDB, CouchDB
Graph Databases: InfoGrid, Neo4j
Other Databases: Kyotocabinet, Berkley DB
All these No-SQL databases, has capability to store unlimited data. These databcase can be used along with Memcached or Ehache for better user exeprience and faster procession.