Cassandra is is a hybrid non-relational database in the same class as Google's BigTable. It is more featureful than a key/value store like Dynomite, but supports fewer query types than a document store like MongoDB. (Quoted from the snax RoR blog). It was open-sourced by Facebook in 2008.
An example application is twissandra. Twissandra is an example project, created to learn and demonstrate how to use Cassandra. Running the project will present a website that has similar functionality to Twitter. The source code of twissandra is found on github, here.
Again, some help on getting started: