Facebook has added an interesting security feature called Trusted Contacts that will help you get back into your account if you ever find yourself locked out. For example, you may have forgotten your password and be unable to access your email. It could also be helpful if someone evil has hacked into your account, and changed your password (but not yet deleted your Trusted Contacts!)
Trusted Contacts are friends that you specify BEFORE you need help. When you do need help accessing your account, your trusted friend will be able to access a one-time code (by visiting a special URL). You will then be able to use this code to verify your identity to Facebook.
To set up your Trusted Contacts, go to Settings / Security and look for the Trusted Contacts setting. After choosing a trusted contact, Facebook will notify your Trusted Contacts of their new role.
To use a Trusted Contact, click on the Forgot Password link on the login page, and follow the instructions.