Scale-Out Cache/Memory

Besides ScaleOut Software's solutions, here is a very popular free solution - memcached.

Memcached's APIs provides a giant hash table distributed across multiple machines. When the table is full, subsequent inserts cause older data to be purged in least recently used (LRU)
order. Applications using memcached typically layer memcached requests
and additions into core before falling back on a slower backing store,
such as a database.

memcached: a distributed memory object caching system