Apache Cassandra is a massively scalable open source NoSQL database. Cassandra is perfect for managing large amounts of structured, semi-structured, and unstructured data across multiple data centers and the cloud. Cassandra delivers linear scalability and performance across many commodity servers with no single point of failure, and provides a powerful dynamic data model designed for maximum flexibility and fast response times.  

Click "Start Here" for a step-by-step guide on how to get up and running with Apache Cassandra.

Already set up and looking for documentation? Check out DataStax's Apache Cassandra 1.2 Documentation, also in PDF format.