There is only one rule of Internet safety that all people know. Never send personal information to suspicious websites. If this is the freshman rule, then there is a sophomore rule as well. Even websites with good intentions do not always succeed at keeping your data safe. For the longest time, I expected this to leave me with absolutely no recourse if the companies in question decide not to publicize their data breaches. However, there is a wonderful service at HaveIBeenPwned.Com which can put some agency back in the hands of us users.
The maintainer, a security expert named Troy Hunt, keeps it up to date with information about hundreds of data breaches on major social networks, blogs and online stores. More ambitiously, you can also search for your email address (or your friend's email address) in order to directly see whether one of the accounts associated with it has been compromised. This is because Troy often obtains the precise databases that are sold by black hat hackers. But the benefits do not stop there. The best part of the site is a function to search for leaked passwords themselves. Whenever Have I Been Pwned knows about one of the passwords you type in, there can be no doubt that it's time to retire that password for good.
This is a function that can be used in three ways so I want to explain all three.