15. A computer-readable data storage device that stores instructions that, when executed by one or more computers of a server system, cause the one or more computers to perform operations comprising: receiving data indicating a first user's rating for a content item; storing data that associates the first user's rating with the content item; receiving, by the one or more computers of the server system, a search query from a device associated with a second user; and providing, by the one or more computers of the server system, search results in response to the search query, the search results indicating the content item and one or more other content items, wherein the search results are selected or ranked based at least in part on the first user's rating for the content item and ratings that other users provided for the content item.
16. The data storage device of claim 15 , wherein receiving data indicating the first user's rating comprises receiving data indicating a rating that the first user provided for a web page; and wherein storing the data that associates the rating with the content item comprises storing that associates an identifier for the web page with the rating that the first user provided for the web page.