This site uses a number of databases that have been released by hackers to the public. The email you enter will NOT be stored, transmitted, or otherwise used beyond this check.
Should I Change My Password is a simple online service which can help identify if you may be at risk.
Visit the site, enter the email address you use for various accounts and click the Check it! button. The service will then search through a large database of compromised account passwords and their associated email addresses to find out if your details are among them. If you are indeed listed, you should change your password right away. If not, you are probably safe for the time being but, of course, there is no absolute guarantee.
It should be noted; Should I Change My Password is not a 100% certain method for ascertaining if your accounts are completely safe, but it is a good way to find out if they may have been compromised. The best and most secure method is to always choose a very strong password in the first place.