OrientDB is a NoSQL DBMS which can store 150,000 documents per second on common hardware. Even with a document-based database, the relationships are managed as in graph databases, with direct connections among records. You can traverse entire or parts of trees and graphs of records in a few milliseconds. It supports schema-less, schema-full, and schema-mixed modes, has a strong security profiling system based on users and roles, and supports SQL between the query languages. Thanks to the SQL layer, it's straightforward to use for people skilled in the relational world.
Ebot is a scalable and distribuited Web crawler. The URLs are saved to a NOSQL database (which supports map/reduce queries) that you can query via RESTful HTTP requests or using your preferred programming languages. The URLs that need to be analyzed are sent to AMQP queues. In this way, it is possible to run several crawlers in parallel and stop and start them without losing URLs.
Plasma implements the map/reduce framework on a compute cluster. It has its own distributed filesystem, PlasmaFS, which is transactional (ACID), reliable, and fast, and which provides a complete set of file operations. PlasmaFS can be accessed via an RPC protocol or via NFS (i.e., it is mountable). Additionally, there is a key/value database on top of PlasmaFS.
Neo4j is a graph database, a fully transactional database that stores data structured as graphs. A graph is a flexible data structure that allows for a more agile and rapid style of development. You can think of Neo4j as a high-performance graph engine with all the features of a mature and robust database. The programmer works with an object-oriented, flexible network structure rather than with strict and static tables, yet enjoys all the benefits of a fully transactional, enterprise-strength database. The community edition is GPLv3 licensed, while the advanced and enterprise editions are AGPLv3 licensed.
Membase is a distributed key-value database management system, optimized for storing data behind interactive Web applications. Membase automatically spreads data and I/O across servers. This "scale out" approach at the data layer permits virtually unlimited growth of transaction capacity, with linear increases in cost and constant per-operation performance. Membase is in production behind thousands of applications, from shared-server deployments at Heroku to dedicated Membase clusters with hundreds of servers at Zynga and NHN.