This question is meant to serve as a list of databases and their configurations that the major web sites use and would be a great reference for anyone thinking of scaling their web site to the size of Twitter, Facebook or even Google.
Please keep your answers to a minimum and be sure to cite any sources used.
EDIT:
Also, please bold both the web-site name and the database for easier scanning.
Currently running 610 (soon to be 1000) Hadoop nodes in a single cluster with Hive datastore. Both Hive and Cassandra have been open-sourced by Facebook.
Facebook stats:
Sources: