Recently Twitter had come up with a list of banned passwords which are commonly used and therefore not allowed for Twitter Accounts. Here is the list: http://blogs.pcmag.com/securitywatch/2009/12/the_twitter_banned_password_li.php

Imperva - a company specializing in Web Application Firewalls has also come up with a similar list. A web application firewall is a firewall with application layer capabilities and logic to block certain application layer attacks, not just the network layer attacks that general firewalls prevent.

Here is the Imperva white paper on passwords: http://www.imperva.com/docs/WP_Consumer_Password_Worst_Practices.pdf