The biggest vulnerability in Internet history was discovered by a Google researcher. This is being called Heartbleed across the Internet. Cliff Note version, a fake packet of information is sent to an SSL website, fake packet is rejected, and sent back with data stored on the servers ram. After this has happened the hacker will get usernames, passwords, & credit card information that was recently used by the server. Because this is hard to track or trace it is hard to say how much data has been stolen. Many sites have applied the patch and recommend you change your password.
Below is a list of patched sites that have been effected and you should change your password NOW! Better safe then sorry.