Check an entered password against the 10,000 most common passwords (From this blog: https://xato.net/passwords/more-top-worst-passwords/#.U5W5hnb5f6U) and return an error if a match is found.
PasswordBannedList
I've made a module for checking a password against a banned list. The list comes from the 10,000 most common password list (https://xato.net/passwords/more-top-worst-passwords/) and when input will produce an error informing the user that their password is in the list and a more secure password should be used.
UPDATE: You can now add a file called banned.list to the site/assets/PasswordBannedList directory and include any passwords you would like to ban inside it, one per line. Please view the built in banned.list if you would like an example of how the file should be written.
Install and use modules at your own risk. Always have a site and database backup before installing new modules.