Hackers hack systems. Their mission: harvest accounts, credentials, e-mail addresses, passwords. It’s a very lucrative business. You don’t want your accounts to be compromised, do you?
PwnedList collects hacked accounts, automatically (from known hacker groups) and manually, i.e. the two-headed team regularly imports dumps of hacked accounts found on the world wide web into their daily updated database. They offer a tool to check whether your account or online identity has been stolen.
Point your browser to https://pwnedlist.com and enter an e-mail address or account name. In seconds you’ll know if the account is listed as compromised.
Not appearing in the list is of course no guarantee that your account is safe but at least if it does, you’ve got a problem. Needless to say that you often need to change passwords and apply a decent level of complexity.