Some of you may have heard about lulzsec and the recent compromises of Sony, PBS, Google and a number of other organizations that have been targeted by them and others recently. Many of these attacks have resulted in the public disclosure of usernames or email addresses and passwords. The best way we can defend ourselves against these breaches is to use different passwords for different sites. If remembering all those passwords is as unmanageable for you as it is for me, try getting a password vault like keepass.
If you want to check if your information has been publicly disclosed, there are a number of sites that have aggregated the data and made it searchable. Try https://shouldichangemypassword.com/. If you find yourself, make sure you change your passwords IMMEDIATELY.