Online Calculators and Charts

Martindales - The Reference Desk - http://www.martindalecenter.com/

The Engineering Toolbox - http://www.engineeringtoolbox.com/material-properties-t_24.html

SI Units - http://physics.nist.gov/cuu/Units/index.html
Essentials of the SI.

Unit converter from Flowmeter Directory - http://www.flowmeterdirectory.com/flowmeter_unit_converter/index.htm
Comprehensive unit converter from Flowmeter directory.

WebElements - http://www.webelements.com/webelements/index.html
Periodic table and charts