Heartbleed : serious vulnerability in open SSL posted April 2014
We have tested some of our own services from attacker’s perspective. We attacked ourselves from outside, without leaving a trace. Without using any privileged information or credentials we were able steal from ourselves the secret keys used for our X.509 certificates, user names and passwords, instant messages, emails and business critical documents and communication.
List of vulnerable websites from the Alexa top 10,000 websites: https://gist.github.com/dberkholz/10169691
You can test a website here: http://filippo.io/Heartbleed/
And also, if you have a lot of time to waste, this random dude seems to know a lot about it :D