Science.gov -- for Scientific Information Only
"Science.gov searches over 40 databases and over 2000 selected websites, offering 200 million pages of authoritative U.S. government science information, including research and development results."
USA.gov -- The U.S. Government's Official Search Engine
"Official web portal contains comprehensive information on government resources
Entrez -- The Life Sciences Search Engine
"Searches across the National Center for Biotechnology Information (NCBI) websites."