Look over WebBandit , an agent that uses various search engines to find, then summarizes hits. Pointcast and Excite are some common multiple engine agents. Perhaps the best rated is the UW's own Metacrawler . Give them all a try on the same search task to see which works best for your needs. There are standard measures of search engine performance.
Some key ones are . . .
Efficiency - or the number of retrieved documents that are useful or fulfill the goals of intended for the search without fetching documents that are false starts or don't fulfill intentions of the search. To the extent that you do, efficiency drops. Recall - or the extent to which all documents fulfilling search requirements are obtained by the engine. To the extent this happens, recall drops. If there are erroneous entries, misspellings, incorrectly identified authors or sponsoring organizations in the database, authority declines.