Astronomical Constants from NASA
Contains astronomical/space constants and equations.
http://www.ksc.nasa.gov/facts/faq04.html

DOE Community
Searchable database for reports that are generated by federally funded projects.
http://www.osti.gov/doecommunity/

Physics and Science Reference by PhysLINK.com

http://www.physlink.com/Reference/Index.cfm

RefScout
A weekly, scientific reference mailing service.
http://www.refscout.com/

The Constants and Equations Pages
Expands reference resources for science, maths, and astronomy.
http://www.scenta.co.uk/tcaep/