I'm not certain how far back this goes, but pressing the Escape key in Safari 5.1.5 pauses animated GIFs on a web page. I have an older Mac with Safari 3.2.1 and this functionality is not present there. It would be great if there were a single command to stop these GIFs completely, but this is better than nothing.
It's possible other browsers have a similar capability, but this appears to be a Safari-specifc shortcut. Testing with iCab 4.8, a browser which uses the built-in WebKit frameworks, does not respond to the same command.
[kirkmc adds: Indeed, this does work. In Firefox, this stops the animation entirely, unlike Safari, where it only pauses the animation. In Chrome and Opera, it has no effect.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120413173748927