As of version 9.0, physical constant functionality is built into Mathematica

Physical Constants Package

In addition to providing a comprehensive environment for calculations and a programming language, Mathematica is also a system for representing and presenting scientific and technical knowledge. Certain packages are included with Mathematica to provide easy access to commonly used scientific data, such as the value of physical constants and conversion factors for various systems of units.

SpeedOfLightthe speed of light is 299792458 Meter/Second
AvogadroConstantAvogadro's constant is approximately 6.02214199×1023Mole^-1
ElectronMassthe mass of an electron is approximately 9.10938188×10-31 Kilogram
FineStructureConstantthe fine structure constant is approximately
EarthMassthe mass of the earth is approximately 5.9742×1024 Kilogram
AccelerationDueToGravitythe acceleration due to gravity is 9.80665 Meter/Second2

Some common physical constants.

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This gives the mass of a proton.
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Here is the distance that light could travel in the age of the universe.
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Physical constants.

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