Cassandra is an open source database distributed, is one of the noteworthy projects of the Apache Software Foundation and appears to be hitting hard. By being distributed enables high availability, fault tolerance and, above all, a highly scalable without loss of performance.
The latest news, coming from a blog interview with Ryan King MyNoSQL is that Twitter is considering migrating their MySQL server clusters Cassandra DB.
This is a product presentation, which can be found in the same web Cassandra Project
It is also interesting blog post from Digg in explaining why they made the first migration to Cassandra.