Realistic password strength estimation:

https://tech.dropbox.com/2012/04/zxcvbn-realistic-password-strength-estimation/

The tool: https://dl.dropbox.com/u/209/zxcvbn/test/index.html

Another website (with added fancy CSS and colours!)

http://howsecureismypassword.net/

Everybody knows correcthorsebatterystaple, ironically(?) I’d hazard a guess to say that correcthorsebatterystaple has most been used so frequently recently that there’s a decent chance it has been added to a dictionary and can be brute forced.