FirstGov for Science

"Science.gov is a gateway to authoritative selected science information provided by U.S. Government agencies, including research and development results."

What does that mean? That means this should be a stop for you when seeking out science related material.

In addition to selected authoritative science Web sites you will also find hard-to-access scientific databases that allow you to grab real time data. This gateway searches across 30 databases and more than 1,700 science Web sites.
Science.gov currently accesses over 47 million pages of government science information.

It is free with no registration required. Pop-up ads are also absent.

Science.gov is an interagency initiative of 17 U.S. government science organizations within 12 Federal agencies.