Many modern browsers support bookmarklets: bookmarks that consist of a JavaScript fragment that gets run when the bookmark is selected. These can do things like automatically completing forms, modifying page styles or searching for information.
I would find it useful if I could specify that a bookmarklet should automatically run when I load a certain page. This would be very useful for fixing up annoying pages: I could make all Mailman moderation forms default to "reject", or automatically disable stylesheets on sites with awkward colour schemes. The page could be specified using a list of patterns to match (in the same way that proxy exceptions are specified).