Half a million widely trusted websites vulnerable to Heartbleed bug

A serious overrun vulnerability in the OpenSSL cryptographic library affects around 17% of SSL web servers which use certificates issued by trusted certificate authorities. Already commonly known as the Heartbleed bug, a missing bounds check in the handling of the TLS heartbeat extension can allow remote attackers to view up to 64 kilobytes of memory on an affected server. This could allow attackers to retrieve private keys and ultimately decrypt the server's encrypted traffic or even impersonate the server.

Bruce Schneier reckons that

On the scale of 1 to 10, this is an 11.

Not the type of things that's nice to see go up to 11...

There is a website allowing to test your servers. It however sometimes seems to return false negative, so be sure to try it multiple times.