etcd is a distributed key-value store, used as shared cluster configuration backend. It automatically elects a master server, allows dynamic config data with optional TTLs, provides a JSON API over HTTP with TLS and long polling, uses the Raft protocol for distribution. It's the basis for CoreOS, integrates with fleet/systemd, and thus also provides service discovery.
Redis is a key-value cache and storage system. It allows for complex data structures, with keys being representable as strings, hashes, lists, sets, or ordered sets, bitmaps, and hyperloglogs.
Apache Drill allows to query non-relational datastores. It supports HBase, MongoDB, MapR-DB, HDFS, MapR-FS, Amazon S3, Azure Blob Storage, Google Cloud Storage, Swift, NAS and local files. The aggregation and query mechanism even allows to join across multiple database backends. The query plan optimizer is datastore-aware and utilizes individual NoSQL filter and query mechanisms depending on each backend scheme.
Scalaris is a distributed and transactional key-value, store with early and full ACID support. Scalaris is implemented in Erlang, uses a structured overlay with a non-blocking Paxos commit protocol for transaction processing with strong consistency over replicas.