Worried that your account details may have been compromised in a hacking attack? PwnedList lets you check if your email account is on a list of compromised accounts, and the site will also perform ongoing monitoring for free.
With security breaches happening frequently, a tool like PwnedList is potentially very useful. The service developers claim it has the world's largest database of stolen credentials.
It's similar to previously mentioned Should I Change My Password?, except PwnedList offers free daily monitoring. Also, when checking your email address manually in PwnedList, you can enter a SHA-512 hash of your email if you don't want to give it to PwnedList. (Online-Convert can generate the hash for you.)
LastPass also recently added a similar feature, LastPass Sentry, that automatically emails you if your email shows up in PwnedList's database. But if you're not using LastPass (or Dashlane, which also offers a similiar option), just use PwnedList directly.