What it is
As the name implies, metasearch engines simply search other search sites, either combining the results into one list or displaying multiple lists on the same web page. These engines provide an overview of search results across multiple sites with the aim of helping users select the most useful search results. 

Who's doing it
Clusty, Dogpile, Mamma, Metacrawler, Vivisimo

Why you should care
Maybe you shouldn't. According to Sullivan, where marketers are concerned, "you can't do anything with metasearch." That is, anything other than employ traditional text or banner ads.