RocksDB is an embedded database for key-value data. It's a fork of LevelDB storing key-value pairs in arbitrary byte arrays. It was started at Facebook on 2012 and now serves as the backend for the interestingly named CockroachDB, the open source DB inspired by Google Spanner.

MongoRocks is a project backed by Percona and Facebook aiming to bring RocksDB backend to MongoDB. RocksDB can achieve higher performance than WiredTiger for some workloads and is worth investigating.