Ok - I saw something groovy on an adult site, and I want to know if anyone knows how to do this -

You know those boxes with screwy letters and numbers that you see on sites like Yahoo or MSN or wherever in order to verify that you are a "real person" ----- well, I am seeing them used on adult sites where to log in you fill in your username, password, and the letters from the box.....

HOW DO I DO THIS ON MY SITES?

For an example lookie here: http://sadoslaves.sweetmembers.com/