How can I use it?

First of all, you can try it out here. Enter your email address into this field and click the button to see what your unicorn looks like (don't worry, your email address won't be sent anywhere, the calculation happens on your computer):

Now, here's how you construct the image URL for a given email address: Take the MD5 hash of the address as a hexadecimal number and append it to http://unicornify.appspot.com/avatar/ – that's it. This will generate a 32x32 pixel image. If you want the large one, append ?s=128 to the URL. The allowed range for the sizes is from 32 to 128 pixels. For example, the image above has the URL

http://unicornify.appspot.com/avatar/ba95799ceedd18316033196cb243cfc0?s=128

That's pretty much identical to the way that Gravatar urls are generated, just with fewer parameters.

Bookmarklet

Arjan van Bentem has created a little bookmarklet that unicornifies the Gravatars on a page. Drag this link to your browser's toolbar:

Unicornify this page

Then, when you're on a page that uses Gravater, just click on that bookmark, and behold: Unicorns!