This is a list of currently missing key/value stores. Please comment if you know another one that you might want to use. Implementations are also greatly appreciated!
Some implementations are not yet finished but are already in the staging branch. They need testing and fixes!
Interesting databases which currently lack a ruby binding, but claim to be very fast:
For inspiration: http://nosql-database.org/, https://github.com/datamapper/dm-core/wiki/Adapters
Asking for support for the neo4j graph database.
Same but for Etcd as it is a resilient Key value store great for multi node shared configuration.
I would suggest looking into Dynamoid for working with AWS DynamoDB. Dynamoid is a near-perfect ActiveRecord drop-in, and I suspect that much of the activerecord adaptor for moneta might work.
FYI promising very fast http://rocksdb.org/ (leveldb fork from Facebook) have a Ruby binding: https://github.com/isamu/rocksdb-ruby
BTW, another incoming actor could be aerospike, http://www.aerospike.com/docs/guide, that's have great ruby client APIs: https://github.com/aerospike/aerospike-client-ruby