A flaw with many major websites - the Heartbleed Bug - means that users of those services will need to update their passwords.
Sensitive account information and passwords may have been available to some with knowledge of the weakness. That vulnerability was only made public on Monday, and websites are starting to be updated.
Until each site has patched the problem away, it's not worth changing all those passwords (which for a lot of us will be a daunting, and time consuming task).
You can check which sites are still vulnerable with this tool, while Mashable has a more user friendly at a glance list. If they've been updated, change your password for that site as soon as possible.
It's worth remembering that the best passwords don't contain reams of special characters, numbers, and uppercase symbols, but instead are really, really long. The upside is that long and uncomplicated passwords are often easy to remember.