While our own LikkitP has created some immortal Sounders gifs, I happen to be of the belief that the more gifs, the better. And browsing Tumblr this evening, I stumbled across a new source of gifs I had no previously known. Now, Tumblr is slightly limited as a gif source as you have to have them at 1MB or lower to post them (Likkit's are regularly much, much larger), some of the best gif-makers can do quite a bit with that 1MB.
Josh Anderson, at icantfeelmyarms.tumblr.com, has made some quite fantastic Sounders gifs, and makes a lot of general MLS-related and other soccer gifs. A few of the best Sounders ones are below (you may need to click them to view) or browse here: http://icantfeelmyarms.tumblr.com/tagged/seattle_sounders, and you can check it out all his gifs here: http://icantfeelmyarms.tumblr.com/tagged/%C3%B8