These tools are useful if you wish to canvas more of the Internet with your search. Since none of the search engines exhaustively index the web, it is sometimes valuable to send your request to more than one search engine to find a relevant article. These tools will simultaneously send your search request to several search engines and collect all the results. This can save you lots of time but your search will be less precise. It will not allow you to tune your search using any of the added features that individual search sites may offer.
There are two tools you can use to perform this type of search: A web site or an application that resides on your computer.