Cassandra, belonging to the NoSQL family of databases, is built for large-scale data storage and retrieval. It presents itself as an alternative to the numerous other relational and non-relational database servers. When you can't choose between scalability and performance, you turn to Cassandra's proven fault-tolerance for your mission-critical data. When it gets serious, you can look to Cassandra's replication across datacenters, and know that come hell or high water, your data is going to be safe.
Altogether, its benefits have made it popular with some of the biggest brands on the web: Netflix, Twitter, eBay, and Reddit, among others.
Lintel loves Cassandra for its versatility and constant availability. We use it for our clients when their database demands approach extreme.
For extra credit, Google "Cassandra" and learn how this project got its intereseting name. After all, it's not every day you get to read about ear-licking snakes.