Note: There is some term confusion in the industry, and two different things are called "Columnar Databases". Some, listed here, are distributed, persistent databases built around the "key-map" data model: all data has a (possibly composite) key, with which a map of key-value pairs is associated. In some systems, multiple such value maps can be associated with a key, and these maps are referred to as "column families" (with value map keys being referred to as "columns").
Another group of technologies that can also be called "columnar databases" is distinguished by how it stores data, on disk or in memory -- rather than storing data the traditional way, where all column values for a given key are stored next to each other, "row by row", these systems store all column values next to each other. So more work is needed to get all columns for a given key, but less work is needed to get all values for a given column.
The former group is referred to as "key map data model" here. The line between these and the Key-value Data Model stores is fairly blurry.
The latter, being more about the storage format than about the data model, is listed under Columnar Databases.
You can read more about this distinction on Prof. Daniel Abadi's blog: Distinguishing two major types of Column Stores.
- Apache Accumulo - distributed key/value store, built on Hadoop.
- Apache Cassandra - column-oriented distributed datastore, inspired by BigTable.
- Apache HBase - column-oriented distributed datastore, inspired by BigTable.
- Baidu Tera - an Internet-scale database, inspired by BigTable.
- Facebook HydraBase - evolution of HBase made by Facebook.
- Google BigTable - column-oriented distributed datastore.
- Google Cloud Datastore - is a fully managed, schemaless database for storing non-relational data over BigTable.
- Hypertable - column-oriented distributed datastore, inspired by BigTable.
- InfiniDB - is accessed through a MySQL interface and use massive parallel processing to parallelize queries.
- Tephra - Transactions for HBase.
- Twitter Manhattan - real-time, multi-tenant distributed database for Twitter scale.
- ScyllaDB - column-oriented distributed datastore written in C++, totally compatible with Apache Cassandra.