Beta-Theta
- Gives general defintions, background and examples from the science fields of biology, physics, chemistry, math, and history. Includes forum boards.
- http://www.beta-theta.com/

EquationSheet.com
- Searchable directory, arranged by field, of equations, constants, symbols and SI units. Also features LaTEX utilities, software links.
- http://www.equationsheet.com/

GrayLIT Network
- Searchable database for reports that are generated by federally funded projects.
- http://www.osti.gov/graylit/

The Constants and Equations Pages
- Expanding reference resource for science, math and astronomy
- http://www.tcaep.co.uk/

RefScout
- A weekly, scientific reference mailing service. Screens databases covering life and medical sciences for the key words or the combination of key words specified and will automatically update with any abstract that has come up during the last seven days.
- http://www.refscout.com/

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