Some of you may have heard of Heartbleed, the web security issue that has affected approximately 67% of all websites.
In a nut-shell Heartbleed is a security issue that was just discovered, which has left the majority of websites using SSL vulnerable for the past 2 years or so. What this means to you, is that whatever username/password you've had on a website affected was readable by 3rd parties who were not supposed to be able to see it. There is no trace left to be able to tell if your information was read at some point or not.
So, what do you do?
1.) If you share the same password on various website (for example, your banking account password is the same as the password on your yahoo account), you should change your passwords immediately. Chances are (67%!!!) that your passwords are not longer as private as you intended them to be.
2.) A good Italian fella has put together this website, which checks actively if the websites you have password protected accounts on are safe or not. You can test such websites here: http://filippo.io/Heartbleed/.
3.) Monitor your credit card and bank statements, and make sure you are familiar with all transactions shown. If not, contact your financial institution immediately.
The biggest names on the affected list are Yahoo and Canada Revenue Agency (currently shut down to address this issue).