One friend of mine told me about this.
Relational databases have a problem on heavy load sites. This project born on Facebook and went open source in 2008.
On their site in one slideshow they compare a performance with 50GB to MySQL, results:
MySQL:
300 ms write
350 ms read
Cassandra:
0.12 ms write
15 ms read
WOW! 
Cassandra Wiki
Cassandra is a highly scalable, eventually consistent, distributed, structured key-value store. Cassandra brings together the distributed systems technologies from Dynamo and the data model from Google’s BigTable. Like Dynamo, Cassandra is eventually consistent. Like BigTable, Cassandra provides a ColumnFamily-based data model richer than typical key/value systems.
Cassandra was open sourced by Facebook in 2008, where it was designed by Avinash Lakshman (one of the authors of Amazon’s Dynamo) and Prashant Malik ( Facebook Engineer ). In a lot of ways you can think of Cassandra as Dynamo 2.0 or a marriage of Dynamo and BigTable. Cassandra is in production use at Facebook but is still under heavy development.
Check it @ Cassandra