You can ask users not to use the same password, you can even encourage things like 1password (too expensive for many people I recommend it to), but what if there was a way to enforce that people registering for your site hadn’t used the same password elsewhere?
It actually wouldn’t be too hard, if you’re registering with firstname.lastname@example.org and the password “abc” when you register and the site hasn’t encrypted and stored the password yet it could try to log into your Gmail account with those details, and if it works force you to choose a different password. There’s no reason this has to be limited to email logins, you could put it against the APIs of WordPress.com, Twitter, Facebook, LinkedIn, any number of other services that expose simple authentication APIs and see where it works. Any successful logins, tell the user they need to pick something else.
Of course all that work and they’ll probably just put a 1 at the end of it.