Clever. TechCrunch writes,

Here’s a handy, easy to remember trick: just add “GIF” to the beginning of the URL. After “www.” and before “youtube.com.”

So, for example, you’d turn www.youtube.com/watch?v=dQw4w9WgXcQ into www.gifyoutube.com/watch?v=dQw4w9WgXcQ and hit enter. Tada!