Feed in a name, user name, email address, etc., it tells you if the password details etc. have been leaked. For example, my email address and login details turn out to have been leaked in the big Adobe hack, which fortunately I already knew so I don't have to start panicking. And my details don't seem to have been leaked in the hack of the NCIS fanfic archives a few months ago, but I've already taken steps about that one and it was high time I changed a few passwords anyway... It doesn't give you any details beyond saying there was a leak, so you can't e.g. feed in someone else's user name and get their password!

https://haveibeenpwned.com/

Well worth a look.

later - if anyone is worried they might be harvesting addresses etc., I got this reply on another site:

Thank you for sharing this site with the group. I just want to confirm that it itself is not a malicious site set up to harvest e-mail addresses and other data. It was mentioned in one of the SANS courses I took for work. (SANS bills itself as "The most trusted source for information security training, certification, and research," and it does live up to its billing.)

https://www.sans.org

It's also recced here

http://www.zdnet.com/article/how-to-find-out-if-your-password-has-been-stolen/