Have you been pwned?
Over the years there have been many large scale data breaches. Hackers have gained access to millions of accounts, often logins and passwords. For each there has been publicity which later dies down. Those details are often traded on from one hack to another.
Too many people are lazy when it comes to user names and passwords. They often use the same details for multiple sites. MySpace, Twitter, Facebook, etc. That is why the hacked accounts have a second life. Hackers will use the login details from one hacked service and try it on another system. Because people are lazy, they will often find easy access.
On this website linked here, are stored a database of email addresses, which have been collected together as account details hacked from websites. These details have been acquired by hackers and certainly sold on.
There is no additional risk for you putting your email address into this website. You will be told if your details appears on their list, and for which site your details were connected. I tried an old email address no longer used and found this entry. The email address was, as correctly identified by the site. Another Email address have listed matches for livespace and Adobe. Of course, I have changed my passwords many times since, and the email addresses, so the knowledge that those email addresses have been used on sites will not be a significant risk. However, too many people use the same password for multiple services. This means that a hacker could gain your details from one hacked service and then ‘try’ your email address and password on other services, and could then gain access.