NoSQL database

Interesting..I am trained in Oracle and have experience in MySQL and PostgreSQL. But when I saw NoSQL database, I am puzzled. 

You can find more information here:
Cassandra
=========
Official website: http://cassandra.apache.org/
Used at: Digg, Facebook, Twitter, Reddit, Rackspace


MongoDB
=======
Official website: http://www.mongodb.org
Used at: ? (do let me know if you have info)

CouchDB
========
Official website: http://couchdb.apache.org/
Online Book for CouchDB (I guess this book is still beta):CouchDB: The Definitive Guide
Used at: ? (do let me know if you have info)

Project Voldemort

=================

Official website: http://project-voldemort.com/

Used at: LinkedIn

I guess there are more NoSQL database available. Then again...I am going to focus on these four. Hopefully by next week, I can run some test on these databases on my Xenserver VM.
Meantime..I have to do some reading.. and re-program my brain from SQL to NoSQL... ...Eventual Consistency.---.Eventually it will be consistent..