A very nasty OpenSSL bug called Heartbleed was made public yesterday. Millions of websites around the world are leaking private information despite using SSL/TLS.
Is your site vulnerable? Find out with Filippo Valsorda's Heartbleed Test.
Remediation steps differ depending on operating system, but with Heartbleed Test we can all make sure we've patched up properly. Thanks Filippo!
UPDATE: Filippo provides a web-based tool to check your site, but we suggest downloading the source and running the tool from your own trusted host. Here's a How To if you need help! (Thanks to André Wendt for the suggestion and Jan Lenhardt for the gist)