From our earlier post on graph databases:

Graph databases apply graph theory to the storage of information about the relationships between entries. The relationships between people in social networks is the most obvious example. The relationships between items and attributes in recommendation engines is another. Yes, it has been noted by many that it's ironic that relational databases aren't good for storing relationship data. Adam Wiggins from Heroku has a lucid explanation of why that is here. Short version: among other things, relationship queries in RDBSes can be complex, slow and unpredictable. Since graph databases are designed for this sort of thing, the queries are more reliable.

Neo4j is an open source, Java based database sponsored by Neo Technologies and is one of the most popular graph databases.

Cassandra is a key-value store database inspired by both Amazon.com's Dynamo and Google's BigTable. It was created at Facebook and is now sponsored by DataStax.