Unless you avoid the news entirely, you've almost certainly heard about the hacking sprees that have been taking place over the past several weeks. Tens of thousands of usernames, passwords and email addresses have been released online for anyone to download. Thankfully, it seems that one kind soul has put all that compromised info to good use.
ShouldIChangeMyPassword.com checks your email address against a database to help you determine whether you should be worried about any of your accounts. You do not need to enter any of your passwords to use this service, nor are any of your passwords stored in their database.