Stylish is a Firefox plugin I use that allows me to have user stylesheets on a per-site basis. That alone wouldn’t be a killer app, but what makes it so special is this: it also has a remote repository of user stylesheets organized by site and available to browse with the click of a mouse.
That functionality, making it a killer app, is handled by userstyles.org. Not only does it allow you to host your user stylesheets there for anybody to use, but it also has integrated updating which makes it easy to keep up to date with the revisions to the site and stylesheet.
I signed up for an account a couple of months ago while looking for this sort of functionality. As my test project I created a stylesheet for Facebook to clean up their new design and increase the amount of screen real estate available for content.
I think that both Stylish and userstyles.org need an online ad campaign to promote their use amongst savvy Firefox users. I think that this is merely the tip of the iceberg for creating entirely new ways for users to experience websites outside of the way the author intended—and sometimes a fresh perspective is needed.