ToroDB is an open source, document-oriented, JSON database that runs on top of PostgreSQL, providing storage and I/O savings and ACID semantics. ToroDB is MongoDB-compatible, so you can use Mongo clients to connect to it.

Riak is a distributed database designed to deliver maximum data availability by distributing data across multiple servers. As long as your client can reach one Riak server, it should be able to write data. In most failure scenarios, the data you want to read should be available, although it may not be the most up-to-date version of that data.

TokuMX is a drop-in replacement for MongoDB, and offers 20X performance improvements, 90% reduction in database size, and support for ACID transactions with MVCC. TokuMX has the same binaries, supports the same drivers, data model, and features of MongoDB, because it shares much of its code with MongoDB.