Facebook has taken a big step forward in its support for OpenID and become a relying partner, meaning that you're now able to log into the site using your OpenID.
Many sites are OpenID providers, including Google, and this move means that you now only have to log in to Google to have Facebook automatically log in too. Facebook is accepting OpenIDs from Yahoo! and MySpace too, though those can't be used for automatic login.
The linking procedure is very simple. You can do it in 30 seconds by logging in to your Google account, visiting your Facebook settings page, clicking "change" next to "Linked accounts" and then approving the link-up.
As mentioned before, many sites provide OpenIDs, but the movement has suffered a little from the reticence of many sites to accept them as logins. Facebook's move will be a big boost for the OpenID foundation. However, it's unclear what this means for Facebook's rival service - Facebook Connect.