Meta Search Engines combine results from multiple search engines.

When you enter a search term into Dogpile, for you will be searching Google, Yahoo, Bing, Ask, About, MIVA, LookSmart and others.

Yippy! (formerly Clusty) scans different search engines and provides a clustering by topics within the results. It also provides details about the numbers of items from the different search engines.

ZapMeta is another metasearch engine to try. It combines results from Yahoo, Microsoft Bing, Cuil, Gigablast, Altavista, Entireweb etc. It includes narrower topics and an easy way to limit by domain type.

There are many other meta search engines (Mamma.com, Surfwax.com, Ixquick.com, etc).

Since Google is the largest search engine, it might be useful to search Google first and then try ZapMeta or Yippy! to cover results from other search engines.