What's a site that only offers "no ads" as their paid-upgrade benefit? I haven't seen one offer just that, it usually comes with a grab-bag of other features that can't be replaced with a client-side script/extension. (DeviantArt's premium account is the upgrade I thought of. (http://sta.sh/047k5sokcr2) The two major client-side features of no ads and more thumbnails are solved with NoScript and AutoPager respectively.)
Off the top of my head, Wowhead offers an ad-free version for premium customers. I guess you also get custom forum avatars for being a premium customer.
The point is, free site that is ad-supported but those go away with a membership - whether you get extras or not.
The Facebook version of Words With Friends sells a 'no-ads' premium pack, but admittedly that's a bit of a different scenario as the ads are in-game modal dialogs that can only be dismissed after X seconds.
