Tumblr is one of my favorite example of effective user interaction model. The sign up form is a masterpiece of simplicity and functionality: no confirmation fields to fill in, no threatening messagges or red alerts for mandatory informations or complicated password policy, just a minimal sets of data, a nice copy on the top, a big cushiony button on the bottom and a discrete, teasing green callout box inviting you eventually to read 24 reasons for loving Tumblr.