For a recent personal project (I won't say which since that would defeat the purpose), I was in need of a new kind of authentication. My wife and I wanted to create a website where we could post photos and news for our family and friends, while keeping that information off of the broader intertubes.  With a baby on the way, we plan on having lots (and lots and lots and lots) of photos to share. Regular, account- or password-based authentication wouldn't work, because I didn't know exactly who would be visiting, so either creating one password and sending it around, or creating passwords for each individual person were not options. So, what I ended up creating was a "do you know me?" authentication system. Here's how it works:

Upon entering the site, you are posed with a challenge.  You can choose your question from a group of 5. In this case, I chose questions that anyone who was part of the family or knew us somehow would be able to answer.  For example, "where did Nick grow up?" or "boy cat's name".  I picked enough different questions, pulling info from enough different aspects of our lives, that anyone who we'd want to get in would be able to get at least one right. In this case, I was using WordPress, so I first created created a series of logins that would correspond to the questions I wanted to ask.  Then, I created a custom login screen using the Themed Login and Registration