Rough day for the internet. A vulnerability was found in the OpenSSL cryptographic library, commonly used to secure Apache and nginx servers. This affects roughly 70% – 80% of servers currently in use. From the Heartbleed Bug site:

The Heartbleed Bug is a serious vulnerability in the popular OpenSSL cryptographic software library. This weakness allows stealing the information protected, under normal conditions, by the SSL/TLS encryption used to secure the Internet. SSL/TLS provides communication security and privacy over the Internet for applications such as web, email, instant messaging (IM) and some virtual private networks (VPNs).

The Heartbleed bug allows anyone on the Internet to read the memory of the systems protected by the vulnerable versions of the OpenSSL software. This compromises the secret keys used to identify the service providers and to encrypt the traffic, the names and passwords of the users and the actual content. This allows attackers to eavesdrop on communications, steal data directly from the services and users and to impersonate services and users.

You can test if your server is vulnerable at Qualys® SSL Labs.

A list of potentially affected sites (that you may consider changing your password on) can be viewed at Digital Trends.

UPDATE 04.12.2014

Additional list of potentially affected sites can be viewed on Mashable.