Here’s a use case. An unregistered visitor visits your blog, and decides that your content is so good that it merits a comment! Congratulations, you’ve fooled them! Now you can twply their account details for $1200 on Sitepoint! Just kidding. But generally, when you visit a blog’s commenting section (such as the Tournology Blog shown above), you’ll see a simple form to authenticate you’re not a spammer, generally including (among other things) an e-mail address field.
Hell, gravatar.com could even use this to improve the user experience of registering your e-mail account. Right now it does a full page refresh and doesn’t even show you a preview!
Easiest form, onblur of email text input field:
Showing all options, again onblur of email text input field.
// integer size: between 1 and 512, default 80 (in pixels)
// maximum rating (in order of raunchiness, least to most): g (default), pg, r, x
// url to define a default image (can also be one of: identicon, monsterid, wavatar)
Update: This script has been moved to Github.