MongoDB—the most popular NoSQL system, especially among startups. A document-oriented database with JSON-like documents in dynamic schemas instead of relational tables that’s used on the back end of sites like Craigslist, eBay, Foursquare. It’s open-source, so it’s free, with good customer service. Read more in Should You Use MongoDB? A Look at the Leading NoSQL Database.
HBase—another Apache project, developed as a part of Hadoop, this open-source, non-relational “column store” NoSQL DB is written in Java, and provides BigTable-like capabilities.
Oracle NoSQL—Oracle’s entry into the NoSQL category.
Apache’s Cassandra DB—born at Facebook, Cassandra is a distributed database that’s great at handling massive amounts of structured data. Anticipate a growing application? Cassandra is excellent at scaling up. Examples: Instagram, Comcast, Apple, and Spotify.
Riak—an open-source key-value store database written in Erlang. It has fault-tolerance replication and automatic data distribution built in for excellent performance.