Earlier today I was looking for a Safari Extensions that could stop animated GIFs, and another to allow me to override CSS on a particular page. After a fruitless search of the official Safari Extensions page, Google turned up both Deanimator and User CSS.
Deanimator takes care of my first request. Namely, it stops animated GIFs dead. Meanwhile, User CSS lets me override a page's CSS. I can use it to change fonts or colors, hide ads and more. Both are quite useful.
The next time you're searching for an extension to complete a particular task, consider checking out the Safari Extensions Tumblr site. That's a great site that keeps growing.
As an aside: I've got 22 Safari extensions installed, and exactly half of them are designed to make the web less annoying.