Google seems the best search engine, in terms of both the number of pages it has surveyed and its ability to deliver first the most relevant pages.

Google logo

          Syntax: " ", natural language.

NorthernLight   To search within your results, add words to your query.     Syntax: " ", +, -, OR, ( ), %, *
Search for 
   Tips
Select a source  Simple Search  |  Power Search

AltaVista
 
Syntax: " ", +, -, *
Search and Display the Results
HotBot  Full graphics version    Syntax: " ", AND, NOT, OR

look for:    
return:    
 Dogpile Search
Web Metasearch Web Catalog Usenet Ftp Newscrawler BizNews
Syntax: " ", ( )

37 Search Engines   (for the despairing ...)         Return to Demo or HTML Guide or types of engines

See also: PubSeach DOE search engine; 21 publishers and 1000 journals.
and also ResearchIndex (NEC free search engine of science, esp., computer science. literature)

several